We report the ability to control relative InN incorporation in InGaN/GaN quantum wells (QWs) grown on the semi-polar and non-polar facets of a core-shell nanorod LED structure by varying the growth conditions. A study of the cathodoluminescence emitted from series of structures with different growth temperatures and pressures for the InGaN QW layer revealed that increasing the growth pressure had the effect of increasing InN incorporation on the semi-polar facets, while increasing the growth temperature improves the uniformity of light emission from the QWs on the non-polar facets.

@Article{strathprints61754,
author = {C. G. Bryce and Le Boulbar, E. D. and P.-M. Coulon and P. R. Edwards and I. G{\^i}rgel and D. W. E. Allsopp and P. A. Shields and R. W. Martin},
title = {Quantum well engineering in InGaN/GaN core-shell nanorod structures},
journal = {Journal of Physics D: Applied Physics},
year = {2017},
month = {September},
abstract = {We report the ability to control relative InN incorporation in InGaN/GaN quantum wells (QWs) grown on the semi-polar and non-polar facets of a core-shell nanorod LED structure by varying the growth conditions. A study of the cathodoluminescence emitted from series of structures with different growth temperatures and pressures for the InGaN QW layer revealed that increasing the growth pressure had the effect of increasing InN incorporation on the semi-polar facets, while increasing the growth temperature improves the uniformity of light emission from the QWs on the non-polar facets.},
keywords = {cathodoluminescence, quantum wells, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/61754/}
}

Advanced structural characterisation techniques which are rapid to use, non-destructive and structurally definitive on the nanoscale are in demand, especially for a detailed understanding of extended-defects and their influence on the properties of materials. We have applied the electron backscatter diffraction (EBSD) technique in a scanning electron microscope to non-destructively characterise and quantify antiphase domains (APDs) in GaP thin films grown on different (001) Si substrates with different offcuts. We were able to image and quantify APDs by relating the asymmetrical intensity distributions observed in the EBSD patterns acquired experimentally and comparing the same with the dynamical electron diffraction simulations. Additionally mean angular error maps were also plotted using automated cross-correlation based approaches to image APDs. Samples grown on substrates with a 4? offcut from the [110] do not show any APDs, whereas samples grown on the exactly oriented substrates contain APDs. The procedures described in our work can be adopted for characterising a wide range of other material systems possessing non-centrosymmetric point groups.

@Article{strathprints61621,
author = {G. Naresh-Kumar and A. Vilalta-Clemente and H. Jussila and A. Winkelmann and G. Nolze and S. Vespucci and S. Nagarajan and A.J. Wilkinson and C. Trager-Cowan},
title = {Quantitative imaging of anti-phase domains by polarity sensitive orientation mapping using electron backscatter diffraction},
journal = {Scientific Reports},
year = {2017},
volume = {7},
pages = {10916},
month = {August},
abstract = {Advanced structural characterisation techniques which are rapid to use, non-destructive and structurally definitive on the nanoscale are in demand, especially for a detailed understanding of extended-defects and their influence on the properties of materials. We have applied the electron backscatter diffraction (EBSD) technique in a scanning electron microscope to non-destructively characterise and quantify antiphase domains (APDs) in GaP thin films grown on different (001) Si substrates with different offcuts. We were able to image and quantify APDs by relating the asymmetrical intensity distributions observed in the EBSD patterns acquired experimentally and comparing the same with the dynamical electron diffraction simulations. Additionally mean angular error maps were also plotted using automated cross-correlation based approaches to image APDs. Samples grown on substrates with a 4? offcut from the [110] do not show any APDs, whereas samples grown on the exactly oriented substrates contain APDs. The procedures described in our work can be adopted for characterising a wide range of other material systems possessing non-centrosymmetric point groups.},
keywords = {quantitative imaging, orientation mapping, thin films, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/61621/}
}

Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar (11-22) Al_xGa_(1-x)N epilayers with AlN contents up to x=0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.

@Article{strathprints61607,
author = {Jochen Bruckbauer and Zhi Li and Naresh Gunasekar and Monika Warzecha and Paul Edwards and Ling Jiu and Jie Bai and Tao Wang and Carol Trager-Cowan and Robert Martin},
title = {Spatially-resolved optical and structural properties of semi-polar (11-22) {AlₓGa₁₋ₓN} with x up to 0.56},
journal = {Scientific Reports},
year = {2017},
volume = {7},
pages = {10804},
month = {August},
abstract = {Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar (11-22) Al\_xGa\_(1-x)N epilayers with AlN contents up to x=0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.},
keywords = {emission wavelengths, III-nitride structures, cathodoluminescence, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/61607/}
}

A metal-organic hybrid perovskite with 3-D framework of metal halide octahedra has been reported as a low-cost, solution processable absorber for a thin film solar cell with a power conversion efficiency over 20%. Low-dimensional layered perovskites with metal halide slabs separated by the insulating organic layers are reported to show higher stability, but the efficiencies of the solar cells are limited by the anisotropy of the crystals because of the confinement of excitons. In order to explore the confinement and transport of excitons in zero-dimensional metal-organic hybrid materials, a highly-orientated film of methylammonium bismuth halide, (CH₃NH₃)₃Bi₂I₉, with a nanometer sized core cluster of Bi₂I₉³⁻ surrounded by insulating CH₃NH₃⁺, was deposited on a quartz substrate via solution processing. The (CH₃NH₃)₃Bi₂I₉ film shows highly anisotropic photoluminescence emission and excitation due to the large proportion of localized excitons coupled with a small number of delocalised excitons from inter-cluster energy transfer. The abrupt increase in photoluminescence quantum yield at excitation energy above twice band gap could indicate a quantum cutting due to the low dimensionality.

@Article{strathprints60928,
author = {Chengsheng Ni and Gordon J. Hedley and Julia Payne and Vladimir Svrcek and Calum McDonald and Lethy Krishnan Jagadamma and Paul Edwards and Robert Martin and Davide Mariotti and Paul Maguire and Ifor Samuel and John Irvine},
title = {Charge carrier localised in zero-dimensional {(CH₃NH₃)₃Bi₂I₉} clusters},
journal = {Nature Communications},
year = {2017},
volume = {8},
pages = {170},
abstract = {A metal-organic hybrid perovskite with 3-D framework of metal halide octahedra has been reported as a low-cost, solution processable absorber for a thin film solar cell with a power conversion efficiency over 20%. Low-dimensional layered perovskites with metal halide slabs separated by the insulating organic layers are reported to show higher stability, but the efficiencies of the solar cells are limited by the anisotropy of the crystals because of the confinement of excitons. In order to explore the confinement and transport of excitons in zero-dimensional metal-organic hybrid materials, a highly-orientated film of methylammonium bismuth halide, (CH₃NH₃)₃Bi₂I₉, with a nanometer sized core cluster of Bi₂I₉³⁻ surrounded by insulating CH₃NH₃⁺, was deposited on a quartz substrate via solution processing. The (CH₃NH₃)₃Bi₂I₉ film shows highly anisotropic photoluminescence emission and excitation due to the large proportion of localized excitons coupled with a small number of delocalised excitons from inter-cluster energy transfer. The abrupt increase in photoluminescence quantum yield at excitation energy above twice band gap could indicate a quantum cutting due to the low dimensionality.},
keywords = {perovskite, metal-organic, metal-halide, octahedra, absorber , thin-film solar cell, photoluminescence quantum yield, Physics, Chemistry, Biochemistry, Genetics and Molecular Biology(all), Chemistry(all), Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/60928/}
}

The structural, morphological, electrical and optical properties of In-rich AlxIn1?xN(0 {\ensuremath{<}} x {\ensuremath{<}} 0.39) layers grown by reactive radio-frequency (RF) sputtering on sapphire areinvestigated as a function of the deposition parameters. The RF power applied to the aluminumtarget (0 W?150 W) and substrate temperature (300 ?C?550 ?C) are varied. X-ray diffractionmeasurements reveal that all samples have a wurtzite crystallographic structure oriented withthe c-axis along the growth direction. The aluminum composition is tuned by changing thepower applied to the aluminum target while keeping the power applied to the indium targetfixed at 40 W. When increasing the Al content from 0 to 0.39, the room-temperature opticalband gap is observed to blue-shift from 1.76 eV to 2.0 eV, strongly influenced by the Burstein?Moss effect. Increasing the substrate temperature, results in an evolution of the morphologyfrom closely-packed columnar to compact. For a substrate temperature of 500 ?C and RFpower for Al of 150 W, compact Al0.39In0.61N films with a smooth surface (root-mean-squaresurface roughness below 1 nm) are produced.

A comparative analysis of free and bound excitons in the photoluminescence (PL) spectra of CuInS2 single crystals grown by the traveling heater (THM) and the chemical vapor transport (CVT) methods is presented. The values of the binding energy of the A free exciton (18.5 and 19.7 meV), determined by measurements of the spectral positions of the ground and excited states, allowed the Bohr radii (3.8 and 3.7 nm), bandgaps (1.5536 and 1.5548 eV) and dielectric constants (10.2 and 9.9) to be calculated for CuInS2 crystals grown by THM and CVT, respectively.

III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by combined top-down etching and regrowth to achieve non-polar sidewalls with a low density of extended defects. While the luminescence is uniform along the non-polar \{1?100\} sidewalls, nano-cathodoluminescence shows a sharp reduction in the luminescent intensity at the intersection of the non-polar \{1?100\} facets. The reduction in the luminescent intensity is accompanied by a reduction in the emission energy localised at the apex of the corners. Correlative compositional analysis reveals an increasing indium content towards the corner except at the apex itself. We propose that the observed variations in the structure and chemistry are responsible for the changes in the optical properties at the corners of the nanorods. The insights revealed by nano-cathodoluminescence will aid in the future development of higher efficiency core-shell nanorods.

@Article{strathprints60877,
author = {J. T. Griffiths and C. X. Ren and P.-M. Coulon and E. D. Le Boulbar and C. G. Bryce and I. Girgel and A. Howkins and I. Boyd and R. W. Martin and D. W. E. Allsopp and P. A. Shields and C. J. Humphreys and R. A. Oliver},
title = {Structural impact on the nanoscale optical properties of InGaN core-shell nanorods},
journal = {Applied Physics Letters},
year = {2017},
volume = {110},
pages = {172105},
month = {April},
abstract = {III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by combined top-down etching and regrowth to achieve non-polar sidewalls with a low density of extended defects. While the luminescence is uniform along the non-polar \{1?100\} sidewalls, nano-cathodoluminescence shows a sharp reduction in the luminescent intensity at the intersection of the non-polar \{1?100\} facets. The reduction in the luminescent intensity is accompanied by a reduction in the emission energy localised at the apex of the corners. Correlative compositional analysis reveals an increasing indium content towards the corner except at the apex itself. We propose that the observed variations in the structure and chemistry are responsible for the changes in the optical properties at the corners of the nanorods. The insights revealed by nano-cathodoluminescence will aid in the future development of higher efficiency core-shell nanorods.},
keywords = {nanorods, light emitting diodes, nano-cathodoluminescence, nitride semiconductors, quantum confined Stark effect, efficiency droop, Optics. Light, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/60877/}
}

In this work, comparative x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements allow a comprehensive characterization of AlₓIn₁₋ₓN thin films grown on GaN. Within the limits of experimental accuracy, and in the compositional range 0.08 < x < 0.28, the lattice parameters of the alloys generally obey Vegard’s rule, varying linearly with the InN fraction. Results are also consistent with the small deviation from linear behaviour suggested by Darakchieva et al (2008 Appl. Phys. Lett. 93 261908). However, unintentional incorporation of Ga, revealed by atom probe tomography (APT) at levels below the detection limit for RBS, may also affect the lattice parameters. Furthermore, in certain samples the compositions determined by XRD and RBS differ significantly. This fact, which was interpreted in earlier publications as an indication of a deviation from Vegard’s rule, may rather be ascribed to the influence of defects or impurities on the lattice parameters of the alloy. The wide-ranging set of AlₓIn₁₋ₓN films studied allowed furthermore a detailed investigation of the composition leading to lattice-matching of AlₓIn₁₋ₓN/GaN bilayers.

@Article{strathprints60823,
author = {S Magalh{\~a}es and N Franco and I M Watson and R W Martin and K P O'Donnell and H P D Schenk and F Tang and T C Sadler and M J Kappers and R A Oliver and T Monteiro and T L Martin and P A J Bagot and M P Moody and E Alves and K Lorenz},
title = {Validity of Vegard's rule for AlₓIn₁₋ₓN (0.08<x<0.28) thin films grown on GaN templates},
journal = {Journal of Physics D: Applied Physics},
year = {2017},
volume = {50},
number = {20},
pages = {205107},
month = {April},
note = {This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics D: Applied Physics. The publisher is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1361-6463/aa69dc},
abstract = {In this work, comparative x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements allow a comprehensive characterization of AlₓIn₁₋ₓN thin films grown on GaN. Within the limits of experimental accuracy, and in the compositional range 0.08 < x < 0.28, the lattice parameters of the alloys generally obey Vegard's rule, varying linearly with the InN fraction. Results are also consistent with the small deviation from linear behaviour suggested by Darakchieva et al (2008 Appl. Phys. Lett. 93 261908). However, unintentional incorporation of Ga, revealed by atom probe tomography (APT) at levels below the detection limit for RBS, may also affect the lattice parameters. Furthermore, in certain samples the compositions determined by XRD and RBS differ significantly. This fact, which was interpreted in earlier publications as an indication of a deviation from Vegard's rule, may rather be ascribed to the influence of defects or impurities on the lattice parameters of the alloy. The wide-ranging set of AlₓIn₁₋ₓN films studied allowed furthermore a detailed investigation of the composition leading to lattice-matching of AlₓIn₁₋ₓN/GaN bilayers.},
keywords = {comparative x-ray diffraction, Rutherford backscattering spectrometry , Vegard's rule, lattice parameters, gallium, thin films, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/60823/}
}

We present an optical spectroscopy study of Cu₂ZnSnSe₄ (CZTSe) thin films deposited on Mo/glass substrates. The [Cu]/[Zn+Sn] ratio in these films varies from nearly stoichiometric to strongly Cu deficient and Zn rich. Increasing Cu deficiency and Zn excess widens the bandgap Eg, determined using photoluminescence excitation (PLE) at 4.2 K, from 0.99 eV to 1.03 eV and blue shifts the dominant band in the photoluminescence (PL) spectra from 0.83 eV to 0.95 eV. The PL spectra of the near stoichiometric film reveal two bands: a dominant band centred at 0.83 eV and a lower intensity one at 0.93 eV. The temperature and excitation intensity dependence of the PL spectra help to identify the recombination mechanisms of the observed emission bands as free-to-bound: recombination of free electrons with holes localised at acceptors affected by randomly distributed potential fluctuations. Both the mean depth of such fluctuations, determined by analysing the shape of the dominant bands, and the broadening energy, estimated from the PLE spectra, become smaller with increasing Cu deficiency and Zn excess which also widens Eg due to an improved ordering of the Cu/Zn atoms. These changes in the elemental composition induce a significant blue shift of the PL bands exceeding the Eg widening. This is attributed to a change of the dominant acceptor for a shallow one, and is beneficial for the solar cell performance. Film regions with a higher degree of Cu/Zn ordering are present in the near stoichiometric film generating the second PL band at 0.93 eV.

@Article{strathprints60524,
author = {M. V. Yakushev and M. A. Sulimov and J. M{\'a}rquez-Prieto and I. Forbes and J. Krustok and P. R. Edwards and V. D. Zhivulko and O. M. Borodavchenko and A. V. Mudryi and R. W. Martin},
title = {Influence of the copper content on the optical properties of {CZTSe} thin films},
journal = {Solar Energy Materials and Solar Cells},
year = {2017},
volume = {168},
pages = {69-77},
month = {April},
abstract = {We present an optical spectroscopy study of Cu₂ZnSnSe₄ (CZTSe) thin films deposited on Mo/glass substrates. The [Cu]/[Zn+Sn] ratio in these films varies from nearly stoichiometric to strongly Cu deficient and Zn rich. Increasing Cu deficiency and Zn excess widens the bandgap Eg, determined using photoluminescence excitation (PLE) at 4.2 K, from 0.99 eV to 1.03 eV and blue shifts the dominant band in the photoluminescence (PL) spectra from 0.83 eV to 0.95 eV. The PL spectra of the near stoichiometric film reveal two bands: a dominant band centred at 0.83 eV and a lower intensity one at 0.93 eV. The temperature and excitation intensity dependence of the PL spectra help to identify the recombination mechanisms of the observed emission bands as free-to-bound: recombination of free electrons with holes localised at acceptors affected by randomly distributed potential fluctuations. Both the mean depth of such fluctuations, determined by analysing the shape of the dominant bands, and the broadening energy, estimated from the PLE spectra, become smaller with increasing Cu deficiency and Zn excess which also widens Eg due to an improved ordering of the Cu/Zn atoms. These changes in the elemental composition induce a significant blue shift of the PL bands exceeding the Eg widening. This is attributed to a change of the dominant acceptor for a shallow one, and is beneficial for the solar cell performance. Film regions with a higher degree of Cu/Zn ordering are present in the near stoichiometric film generating the second PL band at 0.93 eV.},
keywords = {copper, thin films, optical spectroscopy, photoluminescence excitation, stoichiometric film, Cu2ZnSnSe4, defects, zinc, Physics, Surfaces, Coatings and Films, Electronic, Optical and Magnetic Materials, Renewable Energy, Sustainability and the Environment},
url = {http://strathprints.strath.ac.uk/60524/},
}

We analyze the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolor orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modeling of the energy and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channeling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.

@Article{strathprints60424,
author = {Aimo Winkelmann and Gert Nolze and Stefano Vespucci and Naresh Gunasekar and Carol Trager-Cowan and Arantxa Vilalta-Clemente and Angus J. Wilkinson and Maarten Vos},
title = {Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications},
journal = {Journal of Microscopy},
year = {2017},
volume = {267},
number = {3},
pages = {330--346},
month = {March},
abstract = {We analyze the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolor orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modeling of the energy and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channeling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.},
keywords = {electron diffraction, electron microscope, cathodoluminescence, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/60424/}
}

The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing \{1 ?1 0 1\} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing \{1 1 ?2 2\} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109 cm?2 to 3.0 {$\times$} 107 cm?2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.

@Article{strathprints60304,
author = {Le Boulbar, E. D. and J. Priesol and M. Nouf-Allehiani and G. Naresh-Kumar and S. Fox and C. Trager-Cowan and A. {\v S}atka and D. W. E. Allsopp and P. A. Shields},
title = {Design and fabrication of enhanced lateral growth for dislocation reduction in {GaN} using nanodashes},
journal = {Journal of Crystal Growth},
year = {2017},
volume = {466},
pages = {30--38},
month = {May},
abstract = {The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing \{1 ?1 0 1\} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing \{1 1 ?2 2\} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109 cm?2 to 3.0 {$\times$} 107 cm?2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.},
keywords = {defects, metalorganic chemical vapour epitaxy, pendeoepitaxy, selective epitaxy, nitrides, semiconducting III-V materials, gallium nitride, solid-state lighting, cathodoluminescence, electron channelling contrast imaging, Optics. Light, Electrical engineering. Electronics Nuclear engineering, Physics and Astronomy(all), Electrical and Electronic Engineering},
url = {http://strathprints.strath.ac.uk/60304/}
}

We describe a general method to determine the location of a point source of waves relative to a two-dimensional single-crystalline active pixel detector. Based on the inherent structural sensitivity of crystalline sensor materials, characteristic detector diffraction patterns can be used to triangulate the location of a wave emitter. The principle described here can be applied to various types of waves provided that the detector elements are suitably structured. As a prototypical practical application of the general detection principle, a digital hybrid pixel detector is used to localize a source of electrons for Kikuchi diffraction pattern measurements in the scanning electron microscope. This approach provides a promising alternative method to calibrate Kikuchi patterns for accurate measurements of microstructural crystal orientations, strains, and phase distributions.

@Article{strathprints60196,
author = {S. Vespucci and G. Naresh-Kumar and C. Trager-Cowan and K. P. Mingard and D. Maneuski and V. O'Shea and A. Winkelmann},
title = {Diffractive triangulation of radiative point sources},
journal = {Applied Physics Letters},
year = {2017},
volume = {110},
number = {12},
pages = {124103},
month = {March},
abstract = {We describe a general method to determine the location of a point source of waves relative to a two-dimensional single-crystalline active pixel detector. Based on the inherent structural sensitivity of crystalline sensor materials, characteristic detector diffraction patterns can be used to triangulate the location of a wave emitter. The principle described here can be applied to various types of waves provided that the detector elements are suitably structured. As a prototypical practical application of the general detection principle, a digital hybrid pixel detector is used to localize a source of electrons for Kikuchi diffraction pattern measurements in the scanning electron microscope. This approach provides a promising alternative method to calibrate Kikuchi patterns for accurate measurements of microstructural crystal orientations, strains, and phase distributions.},
keywords = {instrumentation, pixel detector, crystalline sensor materials, Physics, Physics and Astronomy (miscellaneous), Radiation},
url = {http://strathprints.strath.ac.uk/60196/}
}

Exciton localization disturbs uniform population inversion, leading to an increase in threshold current for lasing. High Al content AlGaN is required for the fabrication of deep ultra-violet LDs, generating exciton localization. Photoluminescence and cathodoluminescence measurements have been performed on high quality semi-polar (11-22) AlxGa1-xN alloys with high Al composition in order to study the optical properties of both the near-band-edge (NBE) emission and the basal-plane stacking faults (BSFs) related emission, demonstrating different behaviours. Further comparison with the exciton localization of their c-plane counterparts exhibits that the exciton localization in semi-polar (11-22) AlGaN is much smaller than that in c-plane AlGaN.

@Article{strathprints59864,
author = {Z. Li and L. Wang and L. Jiu and J. Bruckbauer and Y. Gong and Y. Zhang and J. Bai and R. W. Martin and T. Wang},
title = {Optical investigation of semi-polar (11-22) {AlₓGa₁₋ₓN} with high {Al} composition},
journal = {Applied Physics Letters},
year = {2017},
volume = {110},
number = {9},
pages = {091102},
month = {February},
abstract = {Exciton localization disturbs uniform population inversion, leading to an increase in threshold current for lasing. High Al content AlGaN is required for the fabrication of deep ultra-violet LDs, generating exciton localization. Photoluminescence and cathodoluminescence measurements have been performed on high quality semi-polar (11-22) AlxGa1-xN alloys with high Al composition in order to study the optical properties of both the near-band-edge (NBE) emission and the basal-plane stacking faults (BSFs) related emission, demonstrating different behaviours. Further comparison with the exciton localization of their c-plane counterparts exhibits that the exciton localization in semi-polar (11-22) AlGaN is much smaller than that in c-plane AlGaN.},
keywords = {exciton localization, uniform population inversion, threshold current, lasing, aluminium, ultra violet laser diodes, photoluminescence, temperature dependence, near-band-edge emission, basal-plane stacking faults, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/59864/}
}

We determine how to alter the properties of the quantum vacuum at ultraviolet wavelengths to simultaneously enhance the spontaneous transition rates and the far field detection rate of quantum emitters. We find the response of several complex nanostructures in the 200 ? 400 nm range, where many organic molecules have fluorescent responses, using an analytic decomposition of the electromagnetic response in terms of continuous spectra of plane waves and discrete sets of modes. Coupling a nanorod with an aluminum substrate gives decay rates up to 2.7 {$\times$} 103 times larger than the decay rate in vacuum and enhancements of 824 for the far field emission into the entire upper semi-space and of 2.04 {$\times$} 103 for emission within a cone with a 60? semi-angle. This effect is due to both an enhancement of the field at the emitter?s position and a reshaping of the radiation patterns near mode resonances and cannot be obtained by replacing the aluminum substrate with a second nanoparticle or with a fused silica substrate. These large decay rates and far field enhancement factors will be very useful in the detection of fluorescence signals, as these resonances can be shifted by changing the dimensions of th nanorod. Moreover, these nanostructures have potential for nano-lasing because the Q factors of these resonances can reach 107.9, higher than the Q factors observed in nano-lasers.

@article{strathprints59693,
volume = {25},
number = {4},
month = {February},
author = {Duncan McArthur and Benjamin Hourahine and Francesco Papoff},
title = {Enhancing ultraviolet spontaneous emission with a designed quantum vacuum},
journal = {Optics Express},
pages = {4162--4179},
year = {2017},
keywords = {subwavelength structures , ultraviolet, fluorescence, fluctuations, relaxations, and noise, Optics. Light, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/59693/},
abstract = {We determine how to alter the properties of the quantum vacuum at ultraviolet wavelengths to simultaneously enhance the spontaneous transition rates and the far field detection rate of quantum emitters. We find the response of several complex nanostructures in the 200 ? 400 nm range, where many organic molecules have fluorescent responses, using an analytic decomposition of the electromagnetic response in terms of continuous spectra of plane waves and discrete sets of modes. Coupling a nanorod with an aluminum substrate gives decay rates up to 2.7 {$\times$} 103 times larger than the decay rate in vacuum and enhancements of 824 for the far field emission into the entire upper semi-space and of 2.04 {$\times$} 103 for emission within a cone with a 60? semi-angle. This effect is due to both an enhancement of the field at the emitter?s position and a reshaping of the radiation patterns near mode resonances and cannot be obtained by replacing the aluminum substrate with a second nanoparticle or with a fused silica substrate. These large decay rates and far field enhancement factors will be very useful in the detection of fluorescence signals, as these resonances can be shifted by changing the dimensions of th nanorod. Moreover, these nanostructures have potential for nano-lasing because the Q factors of these resonances can reach 107.9, higher than the Q factors observed in nano-lasers.}
}

GaN/InGaN core-shell nanorods are promising for optoelectronic applications due to the absence of polarization-related electric fields on the sidewalls, a lower defect density, a larger emission volume and strain relaxation at the free surfaces. The core-shell geometry allows the growth of thicker InGaN shell layers, which would benefit the efficiency of light emitting diodes. However, the growth mode of such layers by metal organic vapor phase epitaxy is poorly understood. Through a combination of nanofabrication, epitaxial growth and detailed characterization, this work reveals an evolution in the growth mode of InGaN epitaxial shells, from a two dimensional (2D) growth mode to three dimensional (3D) striated growth without additional line defect formation with increasing layer thickness. Measurements of the indium distribution show fluctuations along the {\ensuremath{<}}10-10{\ensuremath{>}} directions, with low and high indium composition associated with the 2D and 3D growth modes, respectively. Atomic steps at the GaN/InGaN core-shell interface were observed to occur with a similar frequency as quasi-periodic indium fluctuations along [0001] observed within the 2D layer, to provide evidence that the resulting local strain relief at the steps acts as the trigger for a change of growth mode by elastic relaxation. This study demonstrates that misfit dislocation generation during the growth of wider InGaN shell layers can be avoided by using pre-etched GaN nanorods. Significantly, this enables the growth of absorption-based devices and light-emitting diodes with emissive layers wide enough to mitigate efficiency droop.

@Article{strathprints59627,
author = {Coulon, Pierre-Marie and Shahrzad Hosseini Vajargah and An Bao and Paul R. Edwards and Le Boulbar, Emmanuel D. and Ionut G{\^i}rgel and Robert W. Martin and Colin J. Humphreys and Rachel A. Oliver and Duncan W. E. Allsopp and Philip A. Shields},
title = {Evolution of the m-plane quantum well morphology and composition within a {GaN/InGaN} core-shell structure},
journal = {Crystal Growth and Design},
year = {2017},
volume = {17},
number = {2},
pages = {474--482},
month = {February},
abstract = {GaN/InGaN core-shell nanorods are promising for optoelectronic applications due to the absence of polarization-related electric fields on the sidewalls, a lower defect density, a larger emission volume and strain relaxation at the free surfaces. The core-shell geometry allows the growth of thicker InGaN shell layers, which would benefit the efficiency of light emitting diodes. However, the growth mode of such layers by metal organic vapor phase epitaxy is poorly understood. Through a combination of nanofabrication, epitaxial growth and detailed characterization, this work reveals an evolution in the growth mode of InGaN epitaxial shells, from a two dimensional (2D) growth mode to three dimensional (3D) striated growth without additional line defect formation with increasing layer thickness. Measurements of the indium distribution show fluctuations along the {\ensuremath{<}}10-10{\ensuremath{>}} directions, with low and high indium composition associated with the 2D and 3D growth modes, respectively. Atomic steps at the GaN/InGaN core-shell interface were observed to occur with a similar frequency as quasi-periodic indium fluctuations along [0001] observed within the 2D layer, to provide evidence that the resulting local strain relief at the steps acts as the trigger for a change of growth mode by elastic relaxation. This study demonstrates that misfit dislocation generation during the growth of wider InGaN shell layers can be avoided by using pre-etched GaN nanorods. Significantly, this enables the growth of absorption-based devices and light-emitting diodes with emissive layers wide enough to mitigate efficiency droop.},
keywords = {nanorod, core-shell, InGaN, m-plane, morphology, AFM, TEM, EDX, nanofabrication, epitaxial growth, Chemistry, Physics, Materials Science(all), Chemistry(all), Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/59627/}
}

We describe the development of cross-correlation based high resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging (ECCI), in the scanning electron microscope (SEM), to quantitatively map the strain variation and lattice rotation and determine the density and identify dislocations in nitride semiconductor thin films. These techniques can provide quantitative, rapid, non-destructive analysis of the structural properties of materials with a spatial resolution of order of tens of nanometers. HR-EBSD has a sensitivity to changes of strain and rotation of the order of 10?4 and 0.01? respectively, while ECCI can be used to image single dislocations up to a dislocation density of order 1010 cm?2. In the present work, we report the application of the cross-correlation based HR-EBSD approach to determine the tilt, twist, elastic strain and the distribution and type of threading dislocations in InAlN/AlN/GaN high electron mobility transistor (HEMT) structures grown on two different substrates, namely SiC and sapphire. We describe our procedure to estimate the distribution of geometrically necessary dislocations (GND) based on Nye-Kroner analysis and compare them with the direct imaging of threading dislocations (TDs) by ECCI. Combining data from HR-EBSD and ECCI observations allowed the densities of pure edge, mixed and pure screw threading dislocations to be fully separated.

@Article{strathprints59588,
author = {A. Vilalta-Clemente and G. Naresh-Kumar and M. Nouf-Allehiani and P. Gamarra and M.A. di Forte-Poisson and C. Trager-Cowan and A.J. Wilkinson},
title = {Cross-correlation based high resolution electron backscatter diffraction and electron channelling contrast imaging for strain mapping and dislocation distributions in {InAlN} thin films},
journal = {Acta Materialia},
year = {2017},
volume = {125},
pages = {125--135},
month = {February},
abstract = {We describe the development of cross-correlation based high resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging (ECCI), in the scanning electron microscope (SEM), to quantitatively map the strain variation and lattice rotation and determine the density and identify dislocations in nitride semiconductor thin films. These techniques can provide quantitative, rapid, non-destructive analysis of the structural properties of materials with a spatial resolution of order of tens of nanometers. HR-EBSD has a sensitivity to changes of strain and rotation of the order of 10?4 and 0.01? respectively, while ECCI can be used to image single dislocations up to a dislocation density of order 1010 cm?2. In the present work, we report the application of the cross-correlation based HR-EBSD approach to determine the tilt, twist, elastic strain and the distribution and type of threading dislocations in InAlN/AlN/GaN high electron mobility transistor (HEMT) structures grown on two different substrates, namely SiC and sapphire. We describe our procedure to estimate the distribution of geometrically necessary dislocations (GND) based on Nye-Kroner analysis and compare them with the direct imaging of threading dislocations (TDs) by ECCI. Combining data from HR-EBSD and ECCI observations allowed the densities of pure edge, mixed and pure screw threading dislocations to be fully separated.},
keywords = {EBSD, ECCI, dislocations, InAIN, HEMTs, electron backscatter diffraction, electron channelling contrast imaging, scanning electron microscope, nitride semiconductor thin films, geometrically necessary dislocations, Physics, Polymers and Plastics, Metals and Alloys, Ceramics and Composites, Electronic, Optical and Magnetic Materials},
url = {http://strathprints.strath.ac.uk/59588/}
}

Electron backscatter diffraction (EBSD) is a well-established scanning electron microscope (SEM)-based technique [1]. It allows the non-destructive mapping of the crystal structure, texture, crystal phase and strain with a spatial resolution of tens of nanometers. Conventionally this is performed by placing an electron sensitive screen, typically consisting of a phosphor screen combined with a charge coupled device (CCD) camera, in front of a specimen, usually tilted 70? to the normal of the exciting electron beam. Recently, a number of authors have shown that a significant increase in spatial resolution is achievable when Kikuchi diffraction patterns are acquired in transmission geometry; that is when diffraction patterns are generated by electrons transmitted through an electron-transparent, usually thinned, specimen. The resolution of this technique, called transmission Kikuchi diffraction (TKD), has been demonstrated to be better than 10 nm [2, 3]. We have recently demonstrated the advantages of a direct electron detector, Timepix [4, 5], for the acquisition of standard EBSD patterns [5]. In this article we will discuss the advantages of Timepix to perform TKD and for acquiring spot diffraction patterns and more generally for acquiring scanning transmission electron microscopy micrographs in the SEM. Particularly relevant for TKD, is its very compact size, which allows much more flexibility in the positioning of the detector in the SEM chamber. We will furthermore show recent results using Timepix as a virtual forward scatter detector, and will illustrate the information derivable on producing images through processing of data acquired from different areas of the detector. We will show results from samples ranging from gold nanoparticles to nitride semiconductor nanorods.

@Article{strathprints59555,
author = {S. Vespucci and A. Winkelmann and K. Mingard and D. Maneuski and V. O'Shea and C. Trager-Cowan},
title = {Exploring transmission {K}ikuchi diffraction using a {T}imepix detector},
journal = {Journal of Instrumentation},
year = {2017},
volume = {12},
number = {2},
pages = {C02075},
month = {February},
abstract = {Electron backscatter diffraction (EBSD) is a well-established scanning electron microscope (SEM)-based technique [1]. It allows the non-destructive mapping of the crystal structure, texture, crystal phase and strain with a spatial resolution of tens of nanometers. Conventionally this is performed by placing an electron sensitive screen, typically consisting of a phosphor screen combined with a charge coupled device (CCD) camera, in front of a specimen, usually tilted 70? to the normal of the exciting electron beam. Recently, a number of authors have shown that a significant increase in spatial resolution is achievable when Kikuchi diffraction patterns are acquired in transmission geometry; that is when diffraction patterns are generated by electrons transmitted through an electron-transparent, usually thinned, specimen. The resolution of this technique, called transmission Kikuchi diffraction (TKD), has been demonstrated to be better than 10 nm [2, 3]. We have recently demonstrated the advantages of a direct electron detector, Timepix [4, 5], for the acquisition of standard EBSD patterns [5]. In this article we will discuss the advantages of Timepix to perform TKD and for acquiring spot diffraction patterns and more generally for acquiring scanning transmission electron microscopy micrographs in the SEM. Particularly relevant for TKD, is its very compact size, which allows much more flexibility in the positioning of the detector in the SEM chamber. We will furthermore show recent results using Timepix as a virtual forward scatter detector, and will illustrate the information derivable on producing images through processing of data acquired from different areas of the detector. We will show results from samples ranging from gold nanoparticles to nitride semiconductor nanorods.},
keywords = {radiation, imaging detectors, electron backscatter diffraction, Kikuchi diffraction patterns, transmission Kikuchi diffraction, direct electron detector, Timepix, scanning transmission electron microscopy micrographs, Physics, Instrumentation, Mathematical Physics},
url = {http://strathprints.strath.ac.uk/59555/}
}

Nanoscale surface fissures on AlₓGa₁₋ₓN/GaN (15 nm/1 ?m) heterostructures grown by metalorganic vapour phase epitaxy (MOVPE) were imaged using tapping-mode atomic force microscopy (AFM) and electron channelling contrast imaging (ECCI). Fissure formation was linked to threading dislocations, and was only observed in samples cooled under H2 and NH3, developing with increasing barrier layer Al content. No strain relaxation was detected regardless of fissure formation up to barrier layer Al composition fractions of x = 0.37. A reduction of measured channel carrier density was found in fissured samples at low temperature. This instability is attributed to shallow trap formation associated with fissure boundaries. For Ti/Al/Ni/Au Ohmic contact formation to high Al content barrier layers, fissures were found to offer conduction routes to the 2DEG that allow for low resistance contacts, with fissure-free samples requiring additional optimisation of the metal stack and anneal conditions to achieve contact resistivity of order those measured in fissured samples. In addition, the effects of fissures were found to be detrimental to thermal stability of sheet and contact resistance, suggesting that fissure formation compromises the integrity of the 2DEG.

Detailed knowledge of the dopant concentration and composition of wide band gap AlxGa1?xN layers is of crucial importance for the fabrication of ultra violet (UV) light emitting diodes (LEDs). This paper demonstrates the capabilities of wavelength dispersive X-ray (WDX) spectroscopy in accurately determining these parameters and compares the results with those from high resolution X-ray diffraction (HR-XRD) and secondary ion mass spectrometry (SIMS). WDX spectroscopy has been carried out on different silicon-doped wide bandgap AlxGa1?xN samples (x between 0.80 and 1). This study found a linear increase in the Si concentration with the SiH4/group-III ratio, measuring Si concentrations between 3{$\times$}1018 cm?3 and 2.8{$\times$}1019 cm?3, while no direct correlation between the AlN composition and the Si incorporation ratio was found. Comparison between the composition obtained by WDX and by HR-XRD showed very good agreement in the range investigated, while comparison of the donor concentration between WDX and SIMS found only partial agreement, which we attribute to a number of effects.

@Article{strathprints59282,
author = {Gunnar Kusch and Frank Mehnke and Johannes Enslin and Paul R Edwards and Tim Wernicke and Michael Kneissl and Robert W Martin},
title = {Analysis of doping concentration and composition in wide bandgap {AlGaN:Si} by wavelength dispersive {X}-ray spectroscopy},
journal = {Semiconductor Science and Technology},
year = {2017},
volume = {32},
number = {3},
pages = {035020},
month = {February},
abstract = {Detailed knowledge of the dopant concentration and composition of wide band gap AlxGa1?xN layers is of crucial importance for the fabrication of ultra violet (UV) light emitting diodes (LEDs). This paper demonstrates the capabilities of wavelength dispersive X-ray (WDX) spectroscopy in accurately determining these parameters and compares the results with those from high resolution X-ray diffraction (HR-XRD) and secondary ion mass spectrometry (SIMS). WDX spectroscopy has been carried out on different silicon-doped wide bandgap AlxGa1?xN samples (x between 0.80 and 1). This study found a linear increase in the Si concentration with the SiH4/group-III ratio, measuring Si concentrations between 3{$\times$}1018 cm?3 and 2.8{$\times$}1019 cm?3, while no direct correlation between the AlN composition and the Si incorporation ratio was found. Comparison between the composition obtained by WDX and by HR-XRD showed very good agreement in the range investigated, while comparison of the donor concentration between WDX and SIMS found only partial agreement, which we attribute to a number of effects.},
keywords = {dopant concentration, ultra violet light emitting diodes, wavelength dispersive X-ray spectroscopy, high resolution X-ray diffraction, doping concentrations, semiconductors, dopant composition, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/59282/}
}

Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a `spectator ion’ to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on `switchdown’ from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to (an estimated) several hours at 50 K, 1 mW/cm². Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.

@Article{strathprints59279,
author = {A. K. Singh and K. P. O'Donnell and P. R. Edwards and K. Lorenz and M. J. Kappers and M. Bo{\'c}kowski},
title = {Hysteretic photochromic switching of {Eu-Mg} defects in {GaN} links the shallow transient and deep ground states of the {Mg} acceptor},
journal = {Scientific Reports},
year = {2017},
volume = {7},
pages = {41982},
month = {February},
abstract = {Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a `spectator ion' to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on `switchdown' from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to (an estimated) several hours at 50 K, 1 mW/cm². Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.},
keywords = {photoluminescence spectroscopy, hysteretic photochromic switching, Lany-Zunger model, energy barrier, shallow transient state, deep ground state, acceptor state, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/59279/}
}

GaN-based light emitting diodes (LEDs) have been fabricated on sapphire substrates with different thicknesses of GaN buffer layer grown by a combination of hydride vapor phase epitaxy and metalorganic chemical vapor deposition. We analyzed the LED efficiency and modulation characteristics with buffer thicknesses of 12 ?m and 30 ?m. With the buffer thickness increase, cathodoluminescence hyperspectral imaging shows that the dislocation density in the buffer layer decreases from {$\sim$}1.3X10 8 cm-2 to{$\sim$}1.0 X 10 8 cm-2, and Raman spectra suggest that the compressive stress in the quantum wells is partly relaxed, which leads to a large blue shift in the peak emission wavelength of the photoluminescence and electroluminescent spectra. The combined effects of the low dislocation density and stress relaxation lead to improvements in the efficiency of LEDs with the 30 ?m GaN buffer, but the electrical-to-optical modulation bandwidth is higher for the LEDs with the 12 ?m GaN buffer. A rate equation analysis suggests that defect-related nonradiative recombination can help increase the modulation bandwidth but reduce the LED efficiency at low currents, suggesting that a compromise should be made in the choice of defect density.

@Article{strathprints58427,
author = {Pengfei Tian and Paul R. Edwards and Michael J. Wallace and Robert W. Martin and Jonathan J.D. McKendry and Erdan Gu and Martin D. Dawson and Zhi-Jun Qiu and Chuanyu Jia and Zhizhong Chen and Guoyi Zhang and Lirong Zheng and Ran Liu},
title = {Characteristics of {GaN}-based light emitting diodes with different thicknesses of buffer layer grown by {HVPE} and {MOCVD}},
journal = {Journal of Physics D: Applied Physics},
year = {2017},
volume = {50},
number = {7},
pages = {075101},
month = {January},
abstract = {GaN-based light emitting diodes (LEDs) have been fabricated on sapphire substrates with different thicknesses of GaN buffer layer grown by a combination of hydride vapor phase epitaxy and metalorganic chemical vapor deposition. We analyzed the LED efficiency and modulation characteristics with buffer thicknesses of 12 ?m and 30 ?m. With the buffer thickness increase, cathodoluminescence hyperspectral imaging shows that the dislocation density in the buffer layer decreases from {$\sim$}1.3X10 8 cm-2 to{$\sim$}1.0 X 10 8 cm-2, and Raman spectra suggest that the compressive stress in the quantum wells is partly relaxed, which leads to a large blue shift in the peak emission wavelength of the photoluminescence and electroluminescent spectra. The combined effects of the low dislocation density and stress relaxation lead to improvements in the efficiency of LEDs with the 30 ?m GaN buffer, but the electrical-to-optical modulation bandwidth is higher for the LEDs with the 12 ?m GaN buffer. A rate equation analysis suggests that defect-related nonradiative recombination can help increase the modulation bandwidth but reduce the LED efficiency at low currents, suggesting that a compromise should be made in the choice of defect density.},
keywords = {light emitting diode, GaN, buffer, modulation bandwidth, solid state lighting, Optics. Light, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/58427/}
}

We discuss a scheme for the coherent control of light and plasmons in nanoparticles that have nonlocal dielectric permittivity and contain nonlinear impurities or color centers. We consider particles which have a response to light that is strongly influenced by plasmons over a broad range of frequencies. Our coherent control method enables the reduction of absorption and/or suppression of scattering.

@article{strathprints57291,
volume = {382},
month = {January},
title = {Coherent control of plasmons in nanoparticles with nonlocal response},
author = {D. McArthur and B. Hourahine and F. Papoff},
year = {2017},
pages = {258--265},
journal = {Optics Communications},
keywords = {plasmonics, nanoparticles, nonlocality, optical routing, Optics. Light, Atomic and Molecular Physics, and Optics},
url = {http://strathprints.strath.ac.uk/57291/},
abstract = {We discuss a scheme for the coherent control of light and plasmons in nanoparticles that have nonlocal dielectric permittivity and contain nonlinear impurities or color centers. We consider particles which have a response to light that is strongly influenced by plasmons over a broad range of frequencies. Our coherent control method enables the reduction of absorption and/or suppression of scattering.}
}

With advances in nanolithography and dry etching, top-down methods of nanostructuring have become a widely used tool for improving the efficiency of optoelectronics. These nano dimensions can offer various benefits to the device performance in terms of light extraction and efficiency, but often at the expense of emission color quality. Broadening of the target emission peak and unwanted yellow luminescence are characteristic defect-related effects due to the ion beam etching damage, particularly for III?N based materials. In this article we focus on GaN based nanorods, showing that through thermal annealing the surface roughness and deformities of the crystal structure can be ?self-healed?. Correlative electron microscopy and atomic force microscopy show the change from spherical nanorods to faceted hexagonal structures, revealing the temperature-dependent surface morphology faceting evolution. The faceted nanorods were shown to be strain- and defect-free by cathodoluminescence hyperspectral imaging, micro-Raman, and transmission electron microscopy (TEM). In-situ TEM thermal annealing experiments allowed for real time observation of dislocation movements and surface restructuring observed in ex-situ annealing TEM sampling. This thermal annealing investigation gives new insight into the redistribution path of GaN material and dislocation movement post growth, allowing for improved understanding and in turn advances in optoelectronic device processing of compound semiconductors.

@article{strathprints60967,
volume = {16},
number = {12},
month = {December},
author = {Michelle Conroy and Haoning Li and Vitaly Z. Zubialevich and Gunnar Kusch and Michael Schmidt and Timothy Collins and Colm Glynn and Robert W. Martin and Colm O'Dwyer and Justin D. Holmes and Peter J. Parbrook and Michael D. Morris},
note = {This document is the Accepted Manuscript version of a Published Work that appeared in final form in Crystal Growth and Design, copyright {\copyright} American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.cgd.6b00756},
title = {Self-healing thermal annealing : surface morphological restructuring control of GaN nanorods},
year = {2016},
journal = {Crystal Growth and Design},
pages = {6769--6775},
keywords = {nanolithography, nanostructuring, nanorods, thermal annealing, self-healing, gallium nitride, cathodoluminescence hyperspectral imaging, micro-Raman, Physics, Materials Science(all), Chemistry(all), Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/60967/},
abstract = {With advances in nanolithography and dry etching, top-down methods of nanostructuring have become a widely used tool for improving the efficiency of optoelectronics. These nano dimensions can offer various benefits to the device performance in terms of light extraction and efficiency, but often at the expense of emission color quality. Broadening of the target emission peak and unwanted yellow luminescence are characteristic defect-related effects due to the ion beam etching damage, particularly for III?N based materials. In this article we focus on GaN based nanorods, showing that through thermal annealing the surface roughness and deformities of the crystal structure can be ?self-healed?. Correlative electron microscopy and atomic force microscopy show the change from spherical nanorods to faceted hexagonal structures, revealing the temperature-dependent surface morphology faceting evolution. The faceted nanorods were shown to be strain- and defect-free by cathodoluminescence hyperspectral imaging, micro-Raman, and transmission electron microscopy (TEM). In-situ TEM thermal annealing experiments allowed for real time observation of dislocation movements and surface restructuring observed in ex-situ annealing TEM sampling. This thermal annealing investigation gives new insight into the redistribution path of GaN material and dislocation movement post growth, allowing for improved understanding and in turn advances in optoelectronic device processing of compound semiconductors.}
}

The synthesis and characterisation of two novel organic down-converting molecules is disclosed, together with their performance as functional colour-converters in combination with inorganic blue light-emitting diodes (LEDs). Each molecule contains two fluorene-triphenylamine arms, connected to either a benzothiadiazole or bisbenzothiadiazole core. These molecules have been selected on the basis that they are free from absorption bands in the green region of the visible spectrum to maximise their performance and offer improvements compared with previous BODIPY-containing analogues. The inorganic InGaN/GaN LED emits at 444 nm, overlying the absorption of each of the organic molecules. The combination of the blue (inorganic) and yellow (organic) emission is shown to produce reasonable quality, white light-emitting hybrid devices for both down-converter molecules. Cool to warm white light is achieved for both molecules by increasing the concentration. An optimum colour rendering index (CRI) value of 66 is obtained for the mono-benzothiadiazole molecule. Also a high blue-to-white efficacy (defined as white luminous flux (lm)/blue radiant flux (W)) of 368 lm/W is achieved, superseding the current phosphor converters of 200-300 lm/W. A comparison of these down-converting molecules to the older generation BODIPY-containing molecules is also provided.

@article{strathprints58749,
volume = {4},
number = {48},
month = {November},
author = {Elaine Taylor-Shaw and Enrico Angioni and Neil J. Findlay and Benjamin Breig and Anto R. Inigo and Jochen Bruckbauer and David J. Wallis and Peter J. Skabara and Robert W. Martin},
title = {Cool to warm white light emission from hybrid inorganic/organic light-emitting diodes},
journal = {Journal of Materials Chemistry. C},
pages = {11499--11507},
year = {2016},
keywords = {light-emitting diodes (LEDs), organic down-converting molecules, light emission, c InGaN/GaN LED, Chemistry, Chemistry (miscellaneous)},
url = {http://strathprints.strath.ac.uk/58749/},
abstract = {The synthesis and characterisation of two novel organic down-converting molecules is disclosed, together with their performance as functional colour-converters in combination with inorganic blue light-emitting diodes (LEDs). Each molecule contains two fluorene-triphenylamine arms, connected to either a benzothiadiazole or bisbenzothiadiazole core. These molecules have been selected on the basis that they are free from absorption bands in the green region of the visible spectrum to maximise their performance and offer improvements compared with previous BODIPY-containing analogues. The inorganic InGaN/GaN LED emits at 444 nm, overlying the absorption of each of the organic molecules. The combination of the blue (inorganic) and yellow (organic) emission is shown to produce reasonable quality, white light-emitting hybrid devices for both down-converter molecules. Cool to warm white light is achieved for both molecules by increasing the concentration. An optimum colour rendering index (CRI) value of 66 is obtained for the mono-benzothiadiazole molecule. Also a high blue-to-white efficacy (defined as white luminous flux (lm)/blue radiant flux (W)) of 368 lm/W is achieved, superseding the current phosphor converters of 200-300 lm/W. A comparison of these down-converting molecules to the older generation BODIPY-containing molecules is also provided.}
}

GaN1?xSbx with x{\texttt{\char126}} 5-7\% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photo-electrochemical cell for solar water splitting. In this study, we grew GaN1?xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1?xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb, all suggested that GaN1?xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case, and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions, and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.

@Article{strathprints58170,
author = {Wendy L. Sarney and Stefan P. Svensson and Min Ting and Natalie Segercrantz and Wladek Walukiewicz and Kin Man Yu and Robert W. Martin and Sergei V. Novikov and C.T. Thomas Foxon},
title = {Intermixing studies in {GaN₁₋ₓSbₓ} highly mismatched alloys},
journal = {Applied Optics},
year = {2016},
volume = {56},
number = {3},
pages = {B64--B69},
month = {September},
note = {{\copyright} 2016 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.},
abstract = {GaN1?xSbx with x{\texttt{\char126}} 5-7\% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photo-electrochemical cell for solar water splitting. In this study, we grew GaN1?xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1?xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb, all suggested that GaN1?xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case, and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions, and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.},
keywords = {semiconductor materials, optoelectronics, thin films, Optics. Light, Atomic and Molecular Physics, and Optics},
url = {http://strathprints.strath.ac.uk/58170/}
}

The strong mechanical anisotropy of phosphorene combined with the atomic-scale thickness challenges the commonly employed elastic continuum idealizations. Using objective boundary conditions and a density functional-based potential, we directly uncover the flexibility of individual {\ensuremath{\alpha}}, {\ensuremath{\beta}} and {\ensuremath{\gamma}} phosphorene allotrope layers along an arbitrary bending direction. A correlation analysis with the in-plane elasticity finds that although a monolayer thickness cannot be defined in the classical continuum sense, an unusual orthotropic plate with a directional-dependent thickness can unambiguously describe the out-of-plane deformation of {\ensuremath{\alpha}} and {\ensuremath{\gamma}} allotropes. Such decoupling of the in-plane and out-of-plane nanomechanics might be generic for two-dimensional materials beyond graphene.

White hybrid inorganic/organic light-emitting diodes (LEDs) were fabricated by combining a novel organic colour converter with a blue inorganic LED. An organic small molecule was specifically synthesised to act as down-converter. The characteristics of the white colour were controlled by changing the concentration of the organic molecule based on the BODIPY unit, which was embedded in a transparent matrix, and volume of the molecule and encapsulant mixture. The concentration has a critical effect on the conversion efficiency, i.e. how much of the absorbed blue light is converted into yellow light. With increasing concentration the conversion efficiency decreases. This quenching effect is due to aggregation of the organic molecule at higher concentrations. Increasing the deposited amount of the converter does not increase the yellow emission despite more blue light being absorbed. Degradation of the organic converter was also observed during a period of 15 months from LED fabrication. Angular-dependent measurements revealed slight deviation from a Lambertian profile for the blue and yellow emission peaks leading to a small change in "whiteness" with emission angle. Warm white and cool white light with correlated colour temperatures of 2770 K and 7680 K, respectively, were achieved using different concentrations of the converter molecule. Although further work is needed to improve the lifetime and poor colour rendering, these hybrid LEDs show promising results as an alternative approach for generating white LEDs compared with phosphor-based white LEDs.

@Article{strathprints57454,
author = {Jochen Bruckbauer and Catherine Brasser and Neil J. Findlay and Paul R. Edwards and David J. Wallis and Peter J. Skabara and Robert W. Martin},
title = {Colour tuning in white hybrid inorganic/organic light-emitting diodes},
journal = {Journal of Physics D: Applied Physics},
year = {2016},
volume = {49},
number = {40},
pages = {405103},
month = {September},
abstract = {White hybrid inorganic/organic light-emitting diodes (LEDs) were fabricated by combining a novel organic colour converter with a blue inorganic LED. An organic small molecule was specifically synthesised to act as down-converter. The characteristics of the white colour were controlled by changing the concentration of the organic molecule based on the BODIPY unit, which was embedded in a transparent matrix, and volume of the molecule and encapsulant mixture. The concentration has a critical effect on the conversion efficiency, i.e. how much of the absorbed blue light is converted into yellow light. With increasing concentration the conversion efficiency decreases. This quenching effect is due to aggregation of the organic molecule at higher concentrations. Increasing the deposited amount of the converter does not increase the yellow emission despite more blue light being absorbed. Degradation of the organic converter was also observed during a period of 15 months from LED fabrication. Angular-dependent measurements revealed slight deviation from a Lambertian profile for the blue and yellow emission peaks leading to a small change in "whiteness" with emission angle. Warm white and cool white light with correlated colour temperatures of 2770 K and 7680 K, respectively, were achieved using different concentrations of the converter molecule. Although further work is needed to improve the lifetime and poor colour rendering, these hybrid LEDs show promising results as an alternative approach for generating white LEDs compared with phosphor-based white LEDs.},
keywords = {light-emitting diode, energy efficiencey, organic converter, colour converter, hybrid white LED, inorganic blue LED, Optics. Light, Atomic and Molecular Physics, and Optics, Surfaces, Coatings and Films, Energy Engineering and Power Technology},
url = {http://strathprints.strath.ac.uk/57454/}
}

Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaNxSb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-xSbx HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-xSbx HMA over the entire composition range is well described by a modified the BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaNxSb1-x is general and is applicable to any HMA.

@Article{strathprints56601,
author = {K M Yu and W L Sarney and S V Novikov and N Segercrantz and M Ting and M Shaw and S P Svensson and R W Martin and W Walukiewicz and C T Foxon},
title = {Highly mismatched {GaN₁₋ₓSbₓ} alloys : synthesis, structure and electronic properties},
journal = {Semiconductor Science and Technology},
year = {2016},
volume = {31},
number = {8},
pages = {083001},
month = {June},
abstract = {Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaNxSb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-xSbx HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-xSbx HMA over the entire composition range is well described by a modified the BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaNxSb1-x is general and is applicable to any HMA.},
keywords = {highly mismatched alloys (HMAs), semiconductor alloys, semiconductor synthesis, band anticrossing , Solid state physics. Nanoscience, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/56601/}
}

The inhomogeneous electroluminescence (EL) of InGaN/GaN quantum well light emitting diode structures was investigated in this study. Electroluminescence hyperspectral images showed that inhomogeneities in the form of bright spots exhibited spectrally blue-shifted and broadened emission. Scanning electron microscopy combined with cathodoluminescence (SEM-CL) was used to identify hexagonal pits at the centre of approximately 20\% of these features. Scanning transmission electron microscopy imaging with energy dispersive X-ray spectroscopy (STEM-EDX) indicated there may be p-doped AlGaN within the active region caused by the presence of the pit. Weak beam dark-field TEM (WBDF-TEM) revealed the presence of bundles of dislocations associated with the pit, suggesting the surface features which cause the inhomogeneous EL may occur at coalescence boundaries, supported by trends in the number of features observed across the wafer.

@Article{strathprints56455,
author = {C.X. Ren and B. Rouet-Leduc and J.T. Griffiths and E. Bohacek and M.J. Wallace and P.R. Edwards and M.A. Hopkins and D.W.E. Allsopp and M.J. Kappers and R.W. Martin and R.A. Oliver},
title = {Analysis of defect-related inhomogeneous electroluminescence in {InGaN/GaN QW LED}s},
journal = {Superlattices and Microstructures},
year = {2016},
volume = {99},
pages = {118--124},
abstract = {The inhomogeneous electroluminescence (EL) of InGaN/GaN quantum well light emitting diode structures was investigated in this study. Electroluminescence hyperspectral images showed that inhomogeneities in the form of bright spots exhibited spectrally blue-shifted and broadened emission. Scanning electron microscopy combined with cathodoluminescence (SEM-CL) was used to identify hexagonal pits at the centre of approximately 20\% of these features. Scanning transmission electron microscopy imaging with energy dispersive X-ray spectroscopy (STEM-EDX) indicated there may be p-doped AlGaN within the active region caused by the presence of the pit. Weak beam dark-field TEM (WBDF-TEM) revealed the presence of bundles of dislocations associated with the pit, suggesting the surface features which cause the inhomogeneous EL may occur at coalescence boundaries, supported by trends in the number of features observed across the wafer.},
keywords = {semiconductor, LED, defect, electroluminescence, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/56455/}
}

Controlling the long-range homogeneity of core-shell InGaN/GaN layers is essential for their use in light-emitting devices. This paper demonstrates variations in optical emission energy as low as {\texttt{\char126}}7 meV.?m-1 along the m-plane facets from core-shell InGaN/GaN single quantum wells as measured through high-resolution cathodoluminescence hyperspectral imaging. The layers were grown by metal organic vapor phase epitaxy on etched GaN nanorod arrays with a pitch of 2 ?m. High-resolution transmission electron microscopy and spatially-resolved energy-dispersive X-ray spectroscopy measurements demonstrate a long-range InN-content and thickness homogeneity along the entire 1.2 {\ensuremath{\mu}}m length of the m-plane. Such homogeneous emission was found on the m-plane despite the observation of short range compositional fluctuations in the InGaN single quantum well. The ability to achieve this uniform optical emission from InGaN/GaN core-shell layers is critical to enable them to compete with and replace conventional planar light-emitting devices.

CuInSe2 single crystals ion implanted with 5 keV hydrogen at doses from 3 {$\times$} 1014 to 1016 cm-2 are studied by photoluminescence (PL). The PL spectra before and after implantation reveal two bands, a main dominant band centred at 0.96 eV and a lower intensity band centred at 0.93 eV. Detailed analysis of the shape of these bands, their temperature and excitation intensity dependencies allow the recombination mechanisms to be identified as band-to-tail (BT) and band-to-impurity (BI), respectively. The implantation causes gradual red shifts of the bands increasing linearly with the dose. The average depth of potential fluctuations is also estimated to increase with the dose and saturates for doses above 1015 cm-2. A model is proposed which associates the potential fluctuations with the antisite defects copper on indium site and indium on copper site. The saturation is explained by full randomization of copper and indium atoms on the cation sub-lattice.

@Article{strathprints56024,
author = {M. V. Yakushev and J. Krustok and M. Grossberg and V. A. Volkov and A. V. Mudryi and R. W. Martin},
title = {A photoluminescence study of {CuInSe₂} single crystals ion implanted with 5 {keV} hydrogen},
journal = {Journal of Physics D: Applied Physics},
year = {2016},
volume = {49},
number = {10},
pages = {105108},
month = {February},
abstract = {CuInSe2 single crystals ion implanted with 5 keV hydrogen at doses from 3 {$\times$} 1014 to 1016 cm-2 are studied by photoluminescence (PL). The PL spectra before and after implantation reveal two bands, a main dominant band centred at 0.96 eV and a lower intensity band centred at 0.93 eV. Detailed analysis of the shape of these bands, their temperature and excitation intensity dependencies allow the recombination mechanisms to be identified as band-to-tail (BT) and band-to-impurity (BI), respectively. The implantation causes gradual red shifts of the bands increasing linearly with the dose. The average depth of potential fluctuations is also estimated to increase with the dose and saturates for doses above 1015 cm-2. A model is proposed which associates the potential fluctuations with the antisite defects copper on indium site and indium on copper site. The saturation is explained by full randomization of copper and indium atoms on the cation sub-lattice.},
keywords = {CuInSe, ion-implantation, photoluminescence, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Acoustics and Ultrasonics, Surfaces, Coatings and Films},
url = {http://strathprints.strath.ac.uk/56024/}
}

Stimulated emission and lasing in Cu(In,Ga)Se 2 thin films have been demonstrated at a temperature of 20 K using excitation by a nanosecond pulsed N 2 laser with power densities in the range from 2 to 100 kW cm ? 2 . Sharp narrowing of the photoluminescence band, superlinear dependence of its intensity on excitation laser power, as well as stabilization of the spectral position and of the full-width at half-maximum of the band were observed in the films at increasing excitation intensity. The stimulated emission threshold was determined to be 20 kW cm ? 2 . A gain value of 94 cm ? 1 has been estimated using the variable stripe length method. Several sharp laser modes near 1.13 eV were observed above the laser threshold of I thr {\texttt{\char126}} 50 kW cm ? 2

@Article{strathprints55973,
author = {I E Svitsiankou and V N Pavlovskii and EV Lutsenko and G P Yablonskii and A V Mudryi and VD Zhivulko and M V Yakushev and R W Martin},
title = {Stimulated emission and lasing in {Cu(In,Ga)Se₂} thin films},
journal = {Journal of Physics D: Applied Physics},
year = {2016},
volume = {49},
number = {9},
pages = {095106},
month = {February},
abstract = {Stimulated emission and lasing in Cu(In,Ga)Se 2 thin films have been demonstrated at a temperature of 20 K using excitation by a nanosecond pulsed N 2 laser with power densities in the range from 2 to 100 kW cm ? 2 . Sharp narrowing of the photoluminescence band, superlinear dependence of its intensity on excitation laser power, as well as stabilization of the spectral position and of the full-width at half-maximum of the band were observed in the films at increasing excitation intensity. The stimulated emission threshold was determined to be 20 kW cm ? 2 . A gain value of 94 cm ? 1 has been estimated using the variable stripe length method. Several sharp laser modes near 1.13 eV were observed above the laser threshold of I thr {\texttt{\char126}} 50 kW cm ? 2},
keywords = {stimulated emission, lasing, gain, threshold, photoluminescence, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/55973/}
}

We present structural and optical spectroscopy studies of thin films of Cu2ZnSnSe4 (CZTSe) with strong copper deficiency deposited on Mo/Glass substrates and selenised at 450, 500 or 550 ?C. Solar cells fabricated from these films demonstrated efficiencies up to 7.4\% for selenisation at 500 ?C. Structural analysis based on X-ray diffraction and Raman spectroscopy revealed the presence of SnSe2 in the film selenised at 450 ?C but not detected in the films selenised at higher temperatures. A progressive decrease of the Sn and Se content was observed as the selenisation temperature increased. Photoluminescence excitation was used to determine the bandgaps at 4.2 K. Detailed measurements of the temperature and excitation intensity dependencies of the photoluminescence spectra allow the recombination mechanisms of the observed emission bands to be identified as band-to-impurity and band-to-band transitions, and their evolution with selenisation temperature changes to be analysed. The strongest band-to-band transition is recorded in the PL spectra of the film selenised at 500 ?C and can be observed from 6 K to room temperature. The compositional and structural changes in the films and their influence on the optoelectronic properties of CZTSe and solar cells are discussed.

@Article{strathprints55956,
author = {J. M{\'a}rquez-Prieto and M. V. Yakushev and I. Forbes and J. Krustok and P. R. Edwards and V. D. Zhivulko and O. M. Borodavchenko and A. V. Mudryi and M. Dimitrievska and V. Izquierdo-Roca and N. Pearsall and R. W. Martin},
title = {Impact of the selenisation temperature on the structural and optical properties of {CZTSe} absorbers},
journal = {Solar Energy Materials and Solar Cells},
year = {2016},
volume = {152},
pages = {42--50},
month = {April},
abstract = {We present structural and optical spectroscopy studies of thin films of Cu2ZnSnSe4 (CZTSe) with strong copper deficiency deposited on Mo/Glass substrates and selenised at 450, 500 or 550 ?C. Solar cells fabricated from these films demonstrated efficiencies up to 7.4\% for selenisation at 500 ?C. Structural analysis based on X-ray diffraction and Raman spectroscopy revealed the presence of SnSe2 in the film selenised at 450 ?C but not detected in the films selenised at higher temperatures. A progressive decrease of the Sn and Se content was observed as the selenisation temperature increased. Photoluminescence excitation was used to determine the bandgaps at 4.2 K. Detailed measurements of the temperature and excitation intensity dependencies of the photoluminescence spectra allow the recombination mechanisms of the observed emission bands to be identified as band-to-impurity and band-to-band transitions, and their evolution with selenisation temperature changes to be analysed. The strongest band-to-band transition is recorded in the PL spectra of the film selenised at 500 ?C and can be observed from 6 K to room temperature. The compositional and structural changes in the films and their influence on the optoelectronic properties of CZTSe and solar cells are discussed.},
keywords = {Cu2ZnSnSe4, selenisation, optical spectroscopy, structure, Physics, Surfaces, Coatings and Films, Electronic, Optical and Magnetic Materials, Renewable Energy, Sustainability and the Environment},
url = {http://strathprints.strath.ac.uk/55956/}
}

Core?shell indium gallium nitride (InGaN)/gallium nitride (GaN) structures are attractive as light emitters due to the large nonpolar surface of rod-like cores with their longitudinal axis aligned along the c-direction. These facets do not suffer from the quantum-confined Stark effect that limits the thickness of quantum wells and efficiency in conventional light-emitting devices. Understanding InGaN growth on these submicron three-dimensional structures is important to optimize optoelectronic device performance. In this work, the influence of reactor parameters was determined and compared. GaN nanorods (NRs) with both \{11-20\} a-plane and \{10-10\} m-plane nonpolar facets were prepared to investigate the impact of metalorganic vapor phase epitaxy reactor parameters on the characteristics of a thick (38 to 85 nm) overgrown InGaN shell. The morphology and optical emission properties of the InGaN layers were investigated by scanning electron microscopy, transmission electron microscopy, and cathodoluminescence hyperspectral imaging. The study reveals that reactor pressure has an important impact on the InN mole fraction on the \{10-10\} m-plane facets, even at a reduced growth rate. The sample grown at 750?C and 100 mbar had an InN mole fraction of 25\% on the \{10-10\} facets of the NRs.

@Article{strathprints55885,
author = {Ionut G{\^i}rgel and Paul R. Edwards and Le Boulbar, Emmanuel and Pierre-Marie Coulon and Suman-Lata Sahonta and Duncan W. E. Allsopp and Robert W. Martin and Colin J. Humphreys and Philip A. Shields},
title = {Investigation of indium gallium nitride facet-dependent nonpolar growth rates and composition for core-shell light-emitting diodes},
journal = {Journal of Nanophotonics},
year = {2016},
volume = {10},
number = {1},
pages = {016010},
month = {March},
abstract = {Core?shell indium gallium nitride (InGaN)/gallium nitride (GaN) structures are attractive as light emitters due to the large nonpolar surface of rod-like cores with their longitudinal axis aligned along the c-direction. These facets do not suffer from the quantum-confined Stark effect that limits the thickness of quantum wells and efficiency in conventional light-emitting devices. Understanding InGaN growth on these submicron three-dimensional structures is important to optimize optoelectronic device performance. In this work, the influence of reactor parameters was determined and compared. GaN nanorods (NRs) with both \{11-20\} a-plane and \{10-10\} m-plane nonpolar facets were prepared to investigate the impact of metalorganic vapor phase epitaxy reactor parameters on the characteristics of a thick (38 to 85 nm) overgrown InGaN shell. The morphology and optical emission properties of the InGaN layers were investigated by scanning electron microscopy, transmission electron microscopy, and cathodoluminescence hyperspectral imaging. The study reveals that reactor pressure has an important impact on the InN mole fraction on the \{10-10\} m-plane facets, even at a reduced growth rate. The sample grown at 750?C and 100 mbar had an InN mole fraction of 25\% on the \{10-10\} facets of the NRs.},
keywords = {core-shell, indium gallium nitride, m-plane, a-plane, nonpolar, cathodoluminescence, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/55885/}
}

Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a rapid and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in crystalline materials. In this review, we will demonstrate the application of ECCI to the characterisation of III-nitride semiconductor thin films grown on different substrates and with different crystal orientations. We will briefly describe the history and the theory behind electron channelling and the experimental setup and conditions required to perform ECCI. We will discuss the advantages of using ECCI; especially in combination with other SEM based techniques, such as cathodoluminescence imaging. The challenges in using ECCI are also briefly discussed.

@Article{strathprints55555,
author = {G. Naresh-Kumar and D. Thomson and M. Nouf-Allehiani and J. Bruckbauer and P. R. Edwards and B. Hourahine and R.W. Martin and C. Trager-Cowan},
title = {Electron channelling contrast imaging for {III}-nitride thin film structures},
journal = {Materials Science in Semiconductor Processing},
year = {2016},
volume = {47},
pages = {44--50},
month = {June},
abstract = {Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a rapid and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in crystalline materials. In this review, we will demonstrate the application of ECCI to the characterisation of III-nitride semiconductor thin films grown on different substrates and with different crystal orientations. We will briefly describe the history and the theory behind electron channelling and the experimental setup and conditions required to perform ECCI. We will discuss the advantages of using ECCI; especially in combination with other SEM based techniques, such as cathodoluminescence imaging. The challenges in using ECCI are also briefly discussed.},
keywords = {III - nitrides, extended defects, thin films, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Instrumentation},
url = {http://strathprints.strath.ac.uk/55555/}
}

Eu-doped GaN is a promising material with potential application not only in optoelectronics but also in magneto-optical and quantum optical devices ?beyond the light emitting diode?. Its interesting spectroscopy is unfortunately complicated by spectral overlaps due to ?site multiplicity?, the existence in a given sample of multiple composite centers in which Eu ions associate with intrinsic or extrinsic defects. We show here that elementary crystalfield analysis of the 5D0 to 7F1 transition can critically distinguish such sites. Hence, we find that the center involved in the hysteretic photochromic switching (HPS) observed in GaN(Mg):Eu, proposed as the basis of a new solid state qubit material, is not in fact Eu1, as previously reported, but a related defect, Eu1(Mg). Furthermore, the decomposition of the crystalfield distortions of Eu0, Eu1(Mg) and Eu1 into axial and non-axial components strongly suggests reasonable microscopic models for the defects themselves.

@Article{strathprints55347,
author = {K. P. O'Donnell and P. R. Edwards and M. Yamaga and K. Lorenz and M. J. Kappers and M. Bo{\'c}kowski},
title = {Crystalfield symmetries of luminescent {Eu³⁺} centers in {GaN} : the importance of the {⁵D₀} to {⁷F₁} transition},
journal = {Applied Physics Letters},
year = {2016},
volume = {108},
number = {2},
pages = {022102},
month = {January},
abstract = {Eu-doped GaN is a promising material with potential application not only in optoelectronics but also in magneto-optical and quantum optical devices ?beyond the light emitting diode?. Its interesting spectroscopy is unfortunately complicated by spectral overlaps due to ?site multiplicity?, the existence in a given sample of multiple composite centers in which Eu ions associate with intrinsic or extrinsic defects. We show here that elementary crystalfield analysis of the 5D0 to 7F1 transition can critically distinguish such sites. Hence, we find that the center involved in the hysteretic photochromic switching (HPS) observed in GaN(Mg):Eu, proposed as the basis of a new solid state qubit material, is not in fact Eu1, as previously reported, but a related defect, Eu1(Mg). Furthermore, the decomposition of the crystalfield distortions of Eu0, Eu1(Mg) and Eu1 into axial and non-axial components strongly suggests reasonable microscopic models for the defects themselves.},
keywords = {crystal defects, III-V semiconductors, visible spectra, rare earth ions, emission spectra, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials},
url = {http://strathprints.strath.ac.uk/55347/}
}

The Faraday effect is a representative magneto-optical phenomenon, resulting from the transfer of angular momentum between interacting light and matter in which time-reversal symmetry has been broken by an externally applied magnetic field. Here we report on the Faraday rotation induced in the prominent 3D topological insulator Bi2Se3 due to bulk interband excitations. The origin of this non-resonant effect, extraordinarily strong among other non-magnetic materials, is traced back to the specific Dirac-type Hamiltonian for Bi2Se3, which implies that electrons and holes in this material closely resemble relativistic particles with a non-zero rest mass.

We discuss the impact of kinetics, and in particular the effect of the Ehrlich-Schwöbel barrier (ESB), on the growth and surface morphology of homoepitaxial GaN layers. The presence of an ESB can lead to various self-assembled surface features, which strongly affect the surface roughness. We present an in-depth study of this phenomenon on GaN homoepitaxial layers grown by metal organic vapor phase epitaxy and molecular beam epitaxy. We show how a proper tuning of the growth parameters allows for the control of the surface morphology, independent of the growth technique.

@Article{strathprints53732,
author = {Nils A.K. Kaufmann and L. Lahourcade and B. Hourahine and D. Martin and N. Grandjean},
title = {Critical impact of {E}hrlich-{S}chw{\"o}bel barrier on {GaN} surface morphology during homoepitaxial growth},
journal = {Journal of Crystal Growth},
year = {2016},
volume = {433},
pages = {36--42},
month = {January},
abstract = {We discuss the impact of kinetics, and in particular the effect of the Ehrlich-Schw{\"o}bel barrier (ESB), on the growth and surface morphology of homoepitaxial GaN layers. The presence of an ESB can lead to various self-assembled surface features, which strongly affect the surface roughness. We present an in-depth study of this phenomenon on GaN homoepitaxial layers grown by metal organic vapor phase epitaxy and molecular beam epitaxy. We show how a proper tuning of the growth parameters allows for the control of the surface morphology, independent of the growth technique.},
keywords = {growth models, metalorganic chemical vapor deposition, molecular beam epitaxy, nitrides, semiconducting gallium compounds, surface structure, Physics, Electronic, Optical and Magnetic Materials, Surfaces and Interfaces},
url = {http://strathprints.strath.ac.uk/53732/}
}

Recent developments with group III nitrides present AlxGa1xN based LEDs as realistic devices for new alternative deep ultra-violet light sources. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick wurtzite AlxGa1xN films were grown by PA-MBE on 2-in. GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1xN samples. X-ray microanalysis measurements confirm that the AlN fraction is uniform across the wafer and mass spectroscopy measurements show that the composition is also uniform in depth. We have demonstrated that free-standing wurtzite AlxGa1xN wafers can be achieved by PA-MBE for a wide range of AlN fractions. In order to develop a commercially viable process for the growth of wurtzite AlxGa1xN substrates, we have used a novel Riber plasma source and have demonstrated growth rates of GaN up to 1.8 mm/h on 2-in. diameter GaAs and sapphire wafers

@Article{strathprints56186,
author = {Sergei V. Novikov and C.R. Staddon and Robert Martin and A.J. Kent and C. Thomas Foxon},
title = {Molecular beam epitaxy of free-standing wurtzite {AlₓGa₁₋ₓN} layers},
journal = {Journal of Crystal Growth},
year = {2015},
volume = {425},
pages = {125--128},
month = {February},
abstract = {Recent developments with group III nitrides present AlxGa1xN based LEDs as realistic devices for new alternative deep ultra-violet light sources. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick wurtzite AlxGa1xN films were grown by PA-MBE on 2-in. GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1xN samples. X-ray microanalysis measurements confirm that the AlN fraction is uniform across the wafer and mass spectroscopy measurements show that the composition is also uniform in depth. We have demonstrated that free-standing wurtzite AlxGa1xN wafers can be achieved by PA-MBE for a wide range of AlN fractions. In order to develop a commercially viable process for the growth of wurtzite AlxGa1xN substrates, we have used a novel Riber plasma source and have demonstrated growth rates of GaN up to 1.8 mm/h on 2-in. diameter GaAs and sapphire wafers},
keywords = {substrates, molecular beam epitaxy, nitrides, semiconducting III-V materials, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/56186/}
}

Cu2ZnSnSe4 thin films were synthesised by selenisation of magnetron sputtered metal precursors. The band gap determined from the absorption spectra increases from 1.01 eV at 300 K to 1.05 eV at 4.2 K. In lower quality films photoluminescence spectra show a broad, low intensity asymmetric band associated with a recombination of free electrons and holes localised on acceptors in the presence of spatial potential fluctuations. In high quality material the luminescence band becomes intense and narrow resolving two phonon replicas. Its shifts at changing excitation power suggest donor?acceptor pair recombination mechanisms. The proposed model involving two pairs of donors and acceptors is supported by the evolution of the band intensity and spectral position with temperature. Energy levels of the donors and acceptors are estimated using Arrhenius quenching analysis.

@Article{strathprints55983,
author = {M.V. Yakushev and I. Forbes and A.V. Mudryi and M. Grossberg and J. Krustok and N.S. Beattie and M. Moynihan and A. Rockett and R.W. Martin},
title = {Optical spectroscopy studies of {Cu₂ZnSnSe₄} thin films},
journal = {Thin Solid Films},
year = {2015},
volume = {582},
pages = {154--157},
month = {May},
abstract = {Cu2ZnSnSe4 thin films were synthesised by selenisation of magnetron sputtered metal precursors. The band gap determined from the absorption spectra increases from 1.01 eV at 300 K to 1.05 eV at 4.2 K. In lower quality films photoluminescence spectra show a broad, low intensity asymmetric band associated with a recombination of free electrons and holes localised on acceptors in the presence of spatial potential fluctuations. In high quality material the luminescence band becomes intense and narrow resolving two phonon replicas. Its shifts at changing excitation power suggest donor?acceptor pair recombination mechanisms. The proposed model involving two pairs of donors and acceptors is supported by the evolution of the band intensity and spectral position with temperature. Energy levels of the donors and acceptors are estimated using Arrhenius quenching analysis.},
keywords = {Cu2ZnSnSe4, thin films, photoluminescence, defects, absorption, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/55983/}
}

Ion-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells (MQWs) using 35 keV N+ implantation at room temperature. In situ ion channeling measurements show that damage builds up with a similar trend for In and Ga atoms, with a high threshold for amorphization. The extended defects induced during the implantation, basal and prismatic stacking faults, are uniformly distributed across the quantum well structure. Despite the extremely high fluences used (up to 4 {$\times$}1016 cm-2), the InGaN MQWs exhibit a high stability against ion beam mixing.

High-quality single crystals of CuInSe2 with near-stoichiometric elemental compositions were irradiated with 6 MeV electrons, at doses from 1015 to 3 {$\times$} 1018 cm?2, and studied using photoluminescence (PL) at temperatures from 4.2 to 300 K. Before irradiation, the photoluminescence spectra reveal a number of sharp and well resolved lines associated with free- and bound-excitons. The spectra also show broader bands relating to free-to-bound transitions and their phonon replicas in the lower energy region below 1.0 eV. The irradiation with 6 MeV electrons reduces the intensity of the free- and the majority of the bound-exciton peaks. Such a reduction can be seen for doses above 1016 cm?2. The irradiation induces new PL lines at 1.0215 eV and 0.9909 eV and also enhances the intensity of the lines at 1.0325 and 1.0102 eV present in the photoluminescence spectra before the irradiation. Two broad bands at 0.902 and 0.972 eV, respectively, are tentatively associated with two acceptor-type defects: namely, interstitial selenium (Sei) and copper on indium site (Cu In). After irradiation, these become more intense suggesting an increase in the concentration of these defects due to irradiation.

@Article{strathprints54688,
author = {M. V. Yakushev and A. V. Mudryi and O. M. Borodavchenko and V. A. Volkov and R. W. Martin},
title = {A photoluminescence study of excitonic grade {CuInSe₂} single crystals irradiated with 6 {MeV} electrons},
journal = {Journal of Applied Physics},
year = {2015},
volume = {118},
number = {15},
pages = {155703},
month = {October},
abstract = {High-quality single crystals of CuInSe2 with near-stoichiometric elemental compositions were irradiated with 6 MeV electrons, at doses from 1015 to 3 {$\times$} 1018 cm?2, and studied using photoluminescence (PL) at temperatures from 4.2 to 300 K. Before irradiation, the photoluminescence spectra reveal a number of sharp and well resolved lines associated with free- and bound-excitons. The spectra also show broader bands relating to free-to-bound transitions and their phonon replicas in the lower energy region below 1.0 eV. The irradiation with 6 MeV electrons reduces the intensity of the free- and the majority of the bound-exciton peaks. Such a reduction can be seen for doses above 1016 cm?2. The irradiation induces new PL lines at 1.0215 eV and 0.9909 eV and also enhances the intensity of the lines at 1.0325 and 1.0102 eV present in the photoluminescence spectra before the irradiation. Two broad bands at 0.902 and 0.972 eV, respectively, are tentatively associated with two acceptor-type defects: namely, interstitial selenium (Sei) and copper on indium site (Cu In). After irradiation, these become more intense suggesting an increase in the concentration of these defects due to irradiation.},
keywords = {CuInSe2 , radiation damage, photoluminescence, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/54688/}
}

Thin films of Cu2ZnSnSe4 (CZTSe) with copper de?ficiency and zinc excess were fabricated at Northumbria University by the selenisation of metallic precursors deposited on Mo/glass and bare glass substrates. Absorption and photoluminescence (PL) measurements were used to examine the ?film on glass whereas fi?lms on Mo/glass were used to produce a solar cell with ef?ficiency of 8.1\%. Detailed temperature and excitation intensity analysis of PL spectra allows identifi?cation of the main recombination mechanisms as band-to-tail and band-to-band transitions. The latter transition was observed in the spectra from 6 to 300 K.

@Article{strathprints54657,
author = {M V Yakushev and J M{\'a}rquez-Prieto and I Forbes and P R Edwards and V D Zhivulko and A V Mudryi and J Krustok and R W Martin},
title = {Radiative recombination in {Cu₂ZnSnSe₄} thin films with {Cu} deficiency and {Zn} excess},
journal = {Journal of Physics D: Applied Physics},
year = {2015},
volume = {48},
number = {47},
pages = {475109},
month = {November},
abstract = {Thin films of Cu2ZnSnSe4 (CZTSe) with copper de?ficiency and zinc excess were fabricated at Northumbria University by the selenisation of metallic precursors deposited on Mo/glass and bare glass substrates. Absorption and photoluminescence (PL) measurements were used to examine the ?film on glass whereas fi?lms on Mo/glass were used to produce a solar cell with ef?ficiency of 8.1\%. Detailed temperature and excitation intensity analysis of PL spectra allows identifi?cation of the main recombination mechanisms as band-to-tail and band-to-band transitions. The latter transition was observed in the spectra from 6 to 300 K.},
keywords = {Cu2ZnSnSe4, solar cells, photoluminescence, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/54657/}
}

We model a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. Using exact mathematical formulae, we calculate numerically with a custom fortran code the effect of an external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. Data are provided in tabular, comma delimited value format and illustrate narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts.

@Article{strathprints54610,
author = {Duncan McArthur and Ben Hourahine and Francesco Papoff},
title = {Dataset on coherent control of fields and induced currents in nonlinear multiphoton processes in a nanosphere},
journal = {Scientific Data},
year = {2015},
volume = {2},
pages = {150064},
month = {November},
abstract = {We model a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. Using exact mathematical formulae, we calculate numerically with a custom fortran code the effect of an external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. Data are provided in tabular, comma delimited value format and illustrate narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts.},
keywords = {physical sciences , nonlinear optics, optical physics, nanophotonics and plasmonics physical sciences, Optics. Light, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/54610/}
}

In this work we demonstrate by photoluminescence studies white light emission from a monolithic InGaN/GaN single quantum well structure grown by metal organic chemical vapour deposition. As-grown and thermally annealed samples at high temperature (1000 ?C, 1100 ?C and 1200 ?C) and high pressure (1.1 GPa) were analysed by spectroscopic techniques, and the annealing effect on the photoluminescence is deeply explored. Under laser excitation of 3.8 eV at room temperature, the as-grown structure exhibits two main emission bands: a yellow band peaked at 2.14 eV and a blue band peaked at 2.8 eV resulting in white light perception. Interestingly, the stability of the white light is preserved after annealing at the lowest temperature (1000 ?C), but suppressed for higher temperatures due to a deterioration of the blue quantum well emission. Moreover, the control of the yellow/blue bands intensity ratio, responsible for the white colour coordinate temperatures, could be achieved after annealing at 1000 ?C. The room temperature white emission is studied as a function of incident power density, and the correlated colour temperature values are found to be in the warm white range: 3260?4000 K.

@Article{strathprints54228,
author = {N. Ben Sedrine and T. C. Esteves and J. Rodrigues and L. Rino and M. R. Correia and M. C. Sequeira and A. J. Neves and E. Alves and M. Bo{\'c}kowski and P. R. Edwards and K.P. O'Donnell and K. Lorenz and T. Monteiro},
title = {Photoluminescence studies of a perceived white light emission from a monolithic {InGaN/GaN} quantum well structure},
journal = {Scientific Reports},
year = {2015},
volume = {5},
pages = {13739},
abstract = {In this work we demonstrate by photoluminescence studies white light emission from a monolithic InGaN/GaN single quantum well structure grown by metal organic chemical vapour deposition. As-grown and thermally annealed samples at high temperature (1000 ?C, 1100 ?C and 1200 ?C) and high pressure (1.1 GPa) were analysed by spectroscopic techniques, and the annealing effect on the photoluminescence is deeply explored. Under laser excitation of 3.8 eV at room temperature, the as-grown structure exhibits two main emission bands: a yellow band peaked at 2.14 eV and a blue band peaked at 2.8 eV resulting in white light perception. Interestingly, the stability of the white light is preserved after annealing at the lowest temperature (1000 ?C), but suppressed for higher temperatures due to a deterioration of the blue quantum well emission. Moreover, the control of the yellow/blue bands intensity ratio, responsible for the white colour coordinate temperatures, could be achieved after annealing at 1000 ?C. The room temperature white emission is studied as a function of incident power density, and the correlated colour temperature values are found to be in the warm white range: 3260?4000 K.},
keywords = {light emitting diodes, LEDs, photoluminescence, Physics, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/54228/}
}

Electron backscatter diffraction is a scanning electron microscopy technique used to obtain crystallographic information on materials. It allows the nondestructive mapping of crystal structure, texture, and strain with a lateral and depth resolution on the order of tens of nanometers. Electron backscatter diffraction patterns (EBSPs) are presently acquired using a detector comprising a scintillator coupled to a digital camera, and the crystallographic information obtainable is limited by the conversion of electrons to photons and then back to electrons again. In this article we will report the direct acquisition of energy-filtered EBSPs using a digital complementary metal-oxide-semiconductor hybrid pixel detector, Timepix. We show results from a range of samples with different mass and density, namely diamond, silicon, and GaN. Direct electron detection allows the acquisition of EBSPs at lower ({$\leq$}5 keV) electron beam energies. This results in a reduction in the depth and lateral extension of the volume of the specimen contributing to the pattern and will lead to a significant improvement in lateral and depth resolution. Direct electron detection together with energy filtering (electrons having energy below a specific value are excluded) also leads to an improvement in spatial resolution but in addition provides an unprecedented increase in the detail in the acquired EBSPs. An increase in contrast and higher-order diffraction features are observed. In addition, excess-deficiency effects appear to be suppressed on energy filtering. This allows the fundamental physics of pattern formation to be interrogated and will enable a change in the use of electron backscatter diffraction (EBSD) for crystal phase identification and the mapping of strain. The enhancement in the contrast in high-pass energy-filtered EBSD patterns is found to be stronger for lighter, less dense materials. The improved contrast for such materials will enable the application of the EBSD technique to be expanded to materials for which conventional EBSD analysis is not presently practicable.

@Article{strathprints54220,
author = {S. Vespucci and A. Winkelmann and G. Naresh-Kumar and K. P. Mingard and D. Maneuski and P. R. Edwards and A. P. Day and V. O'Shea and C. Trager-Cowan},
title = {Digital direct electron imaging of energy-filtered electron backscatter diffraction patterns},
journal = {Physical Review B (Condensed Matter)},
year = {2015},
volume = {92},
number = {20},
pages = {205301},
month = {November},
abstract = {Electron backscatter diffraction is a scanning electron microscopy technique used to obtain crystallographic information on materials. It allows the nondestructive mapping of crystal structure, texture, and strain with a lateral and depth resolution on the order of tens of nanometers. Electron backscatter diffraction patterns (EBSPs) are presently acquired using a detector comprising a scintillator coupled to a digital camera, and the crystallographic information obtainable is limited by the conversion of electrons to photons and then back to electrons again. In this article we will report the direct acquisition of energy-filtered EBSPs using a digital complementary metal-oxide-semiconductor hybrid pixel detector, Timepix. We show results from a range of samples with different mass and density, namely diamond, silicon, and GaN. Direct electron detection allows the acquisition of EBSPs at lower ({$\leq$}5 keV) electron beam energies. This results in a reduction in the depth and lateral extension of the volume of the specimen contributing to the pattern and will lead to a significant improvement in lateral and depth resolution. Direct electron detection together with energy filtering (electrons having energy below a specific value are excluded) also leads to an improvement in spatial resolution but in addition provides an unprecedented increase in the detail in the acquired EBSPs. An increase in contrast and higher-order diffraction features are observed. In addition, excess-deficiency effects appear to be suppressed on energy filtering. This allows the fundamental physics of pattern formation to be interrogated and will enable a change in the use of electron backscatter diffraction (EBSD) for crystal phase identification and the mapping of strain. The enhancement in the contrast in high-pass energy-filtered EBSD patterns is found to be stronger for lighter, less dense materials. The improved contrast for such materials will enable the application of the EBSD technique to be expanded to materials for which conventional EBSD analysis is not presently practicable.},
keywords = {electron backscatter diffraction patterns, EBSPs, electron detection, energy filtering, Physics, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/54220/}
}

Temperature-dependent Eu3+ luminescence spectra in GaN(Mg):Eu can be assigned to, at least, two distinct Eu3+ centres, denoted by Eu0 and Eu1. The splitting energy levels of the 7FJ (J=1,2) multiplets for the Eu0 and Eu1 centres have been calculated using the equivalent operator Hamiltonian for C3v crystal field with the addition of an odd parity distortion.

A series of Si-doped AlN-rich AlGaN layers with low resistivities was characterized by a combination of nanoscale imaging techniques. Utilizing the capability of scanning electron microscopy to reliably investigate the same sample area with different techniques, it was possible to determine the effect of doping concentration, defect distribution, and morphology on the luminescence properties of these layers. Cathodoluminescence shows that the dominant defect luminescence depends on the Si-doping concentration. For lower doped samples, the most intense peak was centered between 3.36 eV and 3.39 eV, while an additional, stronger peak appears at 3 eV for the highest doped sample. These peaks were attributed to the (VIII-ON)2? complex and the VIII3? vacancy, respectively. Multimode imaging using cathodoluminescence, secondary electrons, electron channeling contrast, and atomic force microscopy demonstrates that the luminescence intensity of these peaks is not homogeneously distributed but shows a strong dependence on the topography and on the distribution of screw dislocations.

@Article{strathprints54134,
author = {Gunnar Kusch and M. Nouf-Allehiani and Frank Mehnke and Christian Kuhn and Paul R. Edwards and Tim Wernicke and Arne Knauer and Viola Kueller and G. Naresh-Kumar and Markus Weyers and Michael Kneissl and Carol Trager-Cowan and Robert W. Martin},
title = {Spatial clustering of defect luminescence centers in {Si}-doped low resistivity {Al₀.₈₂Ga₀.₁₈N}},
journal = {Applied Physics Letters},
year = {2015},
volume = {107},
number = {7},
pages = {072103},
month = {August},
abstract = {A series of Si-doped AlN-rich AlGaN layers with low resistivities was characterized by a combination of nanoscale imaging techniques. Utilizing the capability of scanning electron microscopy to reliably investigate the same sample area with different techniques, it was possible to determine the effect of doping concentration, defect distribution, and morphology on the luminescence properties of these layers. Cathodoluminescence shows that the dominant defect luminescence depends on the Si-doping concentration. For lower doped samples, the most intense peak was centered between 3.36 eV and 3.39 eV, while an additional, stronger peak appears at 3 eV for the highest doped sample. These peaks were attributed to the (VIII-ON)2? complex and the VIII3? vacancy, respectively. Multimode imaging using cathodoluminescence, secondary electrons, electron channeling contrast, and atomic force microscopy demonstrates that the luminescence intensity of these peaks is not homogeneously distributed but shows a strong dependence on the topography and on the distribution of screw dislocations.},
keywords = {nanoscale imaging, cathodoluminescence, multimode imaging, Physics, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/54134/}
}

In this work we used vertically aligned GaN nanowires with well-defined crystal facets, i.e. the \{11-20\} a-plane, \{10-10\} m-plane, (0001) c-plane and \{1-101\} semi-polar planes, to investigate the impact of MOVPE reactor parameters on the characteristics of an InGaN layer. The morphology and optical characteristics of the InGaN layers grown of each facet were investigated by cathodoluminescence (CL) hyperspectral imaging and scanning electron microscopy (SEM). The influence of reactor parameters on growth rate and alloy fraction were determined and compared. The study revealed that pressure can have an important impact on the incorporation of InN on the \{10-10\} m-plane facets. The growth performed at 750?C and 100mbar led to a homogeneous high InN fraction of 25\% on the \{10-10\} facets of the nanowires. This work suggests homogeneous good quality GaN/InGaN core-shell structure could be grown in the near future.

@Article{strathprints54092,
author = {Ionut G{\^i}rgel and Paul R. Edwards and Emmanuel Le Boulbar and Duncan W. E. Allsopp and Robert W. Martin and Philip A. Shields},
title = {Investigation of facet-dependent {InGaN} growth for core-shell {LED}s},
journal = {Proceedings of SPIE},
year = {2015},
volume = {9363},
pages = {93631V},
month = {March},
note = {Copyright 2015 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.},
abstract = {In this work we used vertically aligned GaN nanowires with well-defined crystal facets, i.e. the \{11-20\} a-plane, \{10-10\} m-plane, (0001) c-plane and \{1-101\} semi-polar planes, to investigate the impact of MOVPE reactor parameters on the characteristics of an InGaN layer. The morphology and optical characteristics of the InGaN layers grown of each facet were investigated by cathodoluminescence (CL) hyperspectral imaging and scanning electron microscopy (SEM). The influence of reactor parameters on growth rate and alloy fraction were determined and compared. The study revealed that pressure can have an important impact on the incorporation of InN on the \{10-10\} m-plane facets. The growth performed at 750?C and 100mbar led to a homogeneous high InN fraction of 25\% on the \{10-10\} facets of the nanowires. This work suggests homogeneous good quality GaN/InGaN core-shell structure could be grown in the near future.},
keywords = {indium gallium nitride, light emitting diodes, nanofibers, indium nitride, gallium nitride, hyperspectral Imaging, scanning electron microscopy, metalorganic chemical vapor deposition, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/54092/}
}

We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.

@Article{strathprints53715,
author = {Daniel Carvalho and Francisco M. Morales and Teresa Ben and Rafael Garc{\'i}a and Andr{\'e}s Redondo-Cubero and Eduardo Alves and Katharina Lorenz and Paul R. Edwards and Kevin P. O'Donnell and Christian Wetzel},
title = {Quantitative chemical mapping of {InGaN} quantum wells from calibrated high-angle annular dark field micrographs},
journal = {Microscopy and Microanalysis},
year = {2015},
volume = {21},
number = {4},
pages = {994--1005},
month = {August},
abstract = {We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.},
keywords = {chemical mapping, energy dispersive X-ray spectroscopy, InGaN/GaN quantum well , HAADF-STEM, RBS, EDX, Physics, Instrumentation},
url = {http://strathprints.strath.ac.uk/53715/}
}

We propose a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. We derive conditions on the external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. The control introduces narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts.

@Article{strathprints53589,
author = {Francesco Papoff and Duncan McArthur and Ben Hourahine},
title = {Coherent control of radiation patterns of nonlinear multiphoton processes in nanoparticles},
journal = {Scientific Reports},
year = {2015},
volume = {5},
pages = {12040},
month = {July},
abstract = {We propose a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. We derive conditions on the external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. The control introduces narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts.},
keywords = {nanoparticles, coherent control, metallic nanospheres, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/53589/}
}

Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1-yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of {$\sim$}15\% was chosen to achieve close lattice matching of Al1-xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1-yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1-xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.

@Article{strathprints53524,
author = {S. Magalh{\~a}es and I. M. Watson and S. Pereira and N. Franco and L. T. Tan and R. W. Martin and K. P. O'Donnell and E. Alves and J. P. Ara{\'u}jo and T. Monteiro and K. Lorenz},
title = {Composition, structure and morphology of {Al1-xInxN} thin films grown on {Al1-yGayN} templates with different {GaN} contents},
journal = {Journal of Physics D: Applied Physics},
year = {2015},
volume = {48},
number = {1},
pages = {015103},
month = {January},
abstract = {Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1-yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of {$\sim$}15\% was chosen to achieve close lattice matching of Al1-xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1-yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1-xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.},
keywords = {AlGaN, AlInN, RBS, XRD, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Acoustics and Ultrasonics, Surfaces, Coatings and Films},
url = {http://strathprints.strath.ac.uk/53524/}
}

Twentyfive years after the publication of P. N. Favennec’s seminal paper on luminescence from rare-earth-doped semiconductors (Electron. Lett. 25, 718-719 (1989), with 390+ citations to date) we examine the long shadow it has cast on recent studies of europium-doped GaN, aimed at substituting for InN-rich InGaN in red-light-emitting devices (LEDs). According to Favennec’s principle, wider band gap semiconductors should show weaker thermal quenching, thus favouring the III-nitrides hugely. The conventional approach to fitting temperature dependences of light emission, based on competition between radiative and non-radiative transitions, is presented here in simplified form and an alternative fitting equation proposed. The original data of Favennec (op. cit.) is re-examined in the light of these fitting models.

We studied the optical properties of metalorganic chemical vapour deposited (MOCVD) InGaN/GaN multiple quantum wells (MQW) subjected to nitrogen (N) implantation and post-growth annealing treatments. The optical characterization was carried out by means of temperature and excitation density-dependent steady state photoluminescence (PL) spectroscopy, supplemented by room temperature PL excitation (PLE) and PL lifetime (PLL) measurements. The as-grown and as-implanted samples were found to exhibit a single green emission band attributed to localized excitons in the QW, although the N implantation leads to a strong reduction of the PL intensity. The green band was found to be surprisingly stable on annealing up to 1400?C. A broad blue band dominates the low temperature PL after thermal annealing in both samples. This band is more intense for the implanted sample, suggesting that defects generated by N implantation, likely related to the diffusion/segregation of indium (In), have been optically activated by the thermal treatment.

@Article{strathprints53469,
author = {Marco A. Sousa and Teresa C. Esteves and Nabiha Ben Sedrine and Joana Rodrigues and M{\'a}rcio B. Louren{\c c}o and Andr{\'e}s Redondo-Cubero and Eduardo Alves and Kevin P. O'Donnell and Michal Bockowski and Christian Wetzel and Maria R. Correia and Katharina Lorenz and Teresa Monteiro},
title = {Luminescence studies on green emitting {InGaN/GaN MQW}s implanted with nitrogen},
journal = {Scientific Reports},
year = {2015},
volume = {5},
pages = {9703},
month = {April},
abstract = {We studied the optical properties of metalorganic chemical vapour deposited (MOCVD) InGaN/GaN multiple quantum wells (MQW) subjected to nitrogen (N) implantation and post-growth annealing treatments. The optical characterization was carried out by means of temperature and excitation density-dependent steady state photoluminescence (PL) spectroscopy, supplemented by room temperature PL excitation (PLE) and PL lifetime (PLL) measurements. The as-grown and as-implanted samples were found to exhibit a single green emission band attributed to localized excitons in the QW, although the N implantation leads to a strong reduction of the PL intensity. The green band was found to be surprisingly stable on annealing up to 1400?C. A broad blue band dominates the low temperature PL after thermal annealing in both samples. This band is more intense for the implanted sample, suggesting that defects generated by N implantation, likely related to the diffusion/segregation of indium (In), have been optically activated by the thermal treatment.},
keywords = {multi quantum wells, luminescence , light emitting diodes, Physics, Optics. Light, General, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/53469/}
}

In this paper we present a combined cathodoluminescence and electron beam induced current study of the optical and electrical properties of InGaN LEDs grown using different active region growth methods. In one device, both the quantum wells and quantum barriers were deposited at their optimum temperatures (2T) whereas in the other device, each barrier was grown in a two step process, with the first few nanometers at a lower temperature (Q2T). It was found that, in the Q2T sample, small micron scale domains of lower emission intensity correlate strongly to a lower EBIC signal, whereas in the 2T sample which has a more uniform emission pattern and an anti-correlation exists between CL emission intensity and EBIC signal.

@Article{strathprints52285,
author = {M. J. Wallace and P. R. Edwards and M. J. Kappers and M. A. Hopkins and F. Oehler and S. Sivaraya and R. A. Oliver and C. J. Humphreys and D. W. E. Allsopp and R. W. Martin},
title = {Effect of the barrier growth mode on the luminescence and conductivity micron scale uniformity of {InGaN} light emitting diodes},
journal = {Journal of Applied Physics},
year = {2015},
volume = {117},
number = {11},
pages = {115705},
abstract = {In this paper we present a combined cathodoluminescence and electron beam induced current study of the optical and electrical properties of InGaN LEDs grown using different active region growth methods. In one device, both the quantum wells and quantum barriers were deposited at their optimum temperatures (2T) whereas in the other device, each barrier was grown in a two step process, with the first few nanometers at a lower temperature (Q2T). It was found that, in the Q2T sample, small micron scale domains of lower emission intensity correlate strongly to a lower EBIC signal, whereas in the 2T sample which has a more uniform emission pattern and an anti-correlation exists between CL emission intensity and EBIC signal.},
keywords = {cathodoluminescence, light emitting diode, electron beams, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/52285/}
}

The electronic structure of the solar cell absorber CuInSe2 is studied using magneto-transmission in thin polycrystalline films at magnetic fields up to 29 T. A, B, and C free excitons are resolved in absorption spectra at zero field and a Landau level fan generated by diamagnetic exciton recombination is observed for fields above 7 T. The dependence of the C band exciton binding energy on magnetic fields, calculated using a hydrogenic approximation, is used to determine the C exciton Rydberg at 0 T (8.5 meV), band gap (1.2828 eV), and hole effective mass mso = (0.31 {$\pm$} 0.12)m0 for the C valence sub-band.

We report on the consistent measurement of gallium incorporation in nominal InAlN layers using various complimentary techniques, underpinned by X-ray diffraction. Nominal InAlN layers with similar growth conditions were prepared, and the change in unintended Ga content in the group III sublattice ranged from similar to 24\% to similar to 12\% when the total reactor flow rate was increased from 8000 to 24 000 standard cubic centimetres per minute. Ultra-thin InAlN/GaN HEMT layers were grown in a clean reactor to minimize Ga auto-incorporation, and measured using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The implications of Ga incorporation in InAlN layers within optoelectronic and power devices is discussed.

@article{strathprints51372,
volume = {2},
number = {29},
title = {Determination of Ga auto-incorporation in nominal InAlN epilayers grown by MOCVD},
author = {M. D. Smith and E. Taylor and T. C. Sadler and V. Z. Zubialevich and K. Lorenz and H. N. Li and J. O'Connell and E. Alves and J. D. Holmes and R. W. Martin and P. J. Parbrook},
year = {2014},
pages = {5787--5792},
journal = {Journal of Materials Chemistry. C},
keywords = {auto-incorporation, epilayers, gallium, InAlN, GaN, Physical and theoretical chemistry, Solid state physics. Nanoscience, Physical and Theoretical Chemistry},
url = {http://strathprints.strath.ac.uk/51372/},
abstract = {We report on the consistent measurement of gallium incorporation in nominal InAlN layers using various complimentary techniques, underpinned by X-ray diffraction. Nominal InAlN layers with similar growth conditions were prepared, and the change in unintended Ga content in the group III sublattice ranged from similar to 24\% to similar to 12\% when the total reactor flow rate was increased from 8000 to 24 000 standard cubic centimetres per minute. Ultra-thin InAlN/GaN HEMT layers were grown in a clean reactor to minimize Ga auto-incorporation, and measured using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The implications of Ga incorporation in InAlN layers within optoelectronic and power devices is discussed.}
}

A systematic investigation on the effects of growth temperature, Ga flux, and Sb flux on the incorporation of Sb, film structure, and optical properties of the GaN1-xSbx highly mismatched alloys (HMAs) was carried out. We found that the direct bandgap ranging from 3.4 eV to below 1.0 eV for the alloys grown at low temperature. At the growth temperature of 80 degrees C, GaN1-xSbx with x{\ensuremath{>}}6\% losses crystallinity and becomes primarily amorphous with small crystallites of 2-5 nm. Despite the range of microstructures found for GaN1-xSbx alloys with different composition, a well-developed absorption edge shifts from 3.4 eV (GaN) to close to 2 eV for samples with a small amount, less than 10\% of Sb. Luminescence from dilute GaN1-xSbx alloys grown at high temperature and the bandgap energy for alloys with higher Sb content are consistent with a localized substitutional Sb level E-Sb at similar to 1.1 eV above the valence band of GaN. The decrease in the bandgap of GaN1-xSbx HMAs is consistent with the formation of a Sb-derived band due to the anticrossing interaction of the Sb states with the valence band of GaN.

@Article{strathprints51371,
author = {K.M. Yu and S. V. Novikov and Min Ting and W.L. Sarney and S.P. Svensson and M. Shaw and R.W. Martin and W. Walukiewicz and C.T. Foxon},
title = {Growth and characterization of highly mismatched {GaN₁₋ₓSbₓ} alloys},
journal = {Journal of Applied Physics},
year = {2014},
volume = {116},
number = {12},
pages = {123704},
month = {September},
note = {{\copyright} 2015 AIP Publishing},
abstract = {A systematic investigation on the effects of growth temperature, Ga flux, and Sb flux on the incorporation of Sb, film structure, and optical properties of the GaN1-xSbx highly mismatched alloys (HMAs) was carried out. We found that the direct bandgap ranging from 3.4 eV to below 1.0 eV for the alloys grown at low temperature. At the growth temperature of 80 degrees C, GaN1-xSbx with x{\ensuremath{>}}6\% losses crystallinity and becomes primarily amorphous with small crystallites of 2-5 nm. Despite the range of microstructures found for GaN1-xSbx alloys with different composition, a well-developed absorption edge shifts from 3.4 eV (GaN) to close to 2 eV for samples with a small amount, less than 10\% of Sb. Luminescence from dilute GaN1-xSbx alloys grown at high temperature and the bandgap energy for alloys with higher Sb content are consistent with a localized substitutional Sb level E-Sb at similar to 1.1 eV above the valence band of GaN. The decrease in the bandgap of GaN1-xSbx HMAs is consistent with the formation of a Sb-derived band due to the anticrossing interaction of the Sb states with the valence band of GaN.},
keywords = {GaN, GaSb, HMAs, film structures, growth temperature, Ga flux, Sb flux, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/51371/}
}

In this paper we report our study on n-type Te doping of amorphous GaNi1-xAsx layers grown by plasma assisted molecular beam epitaxy. We have used a low temperature PbTe source as a source of tellurium. Reproducible and uniform tellurium incorporation in amorphous GaNi1-xAsx layers has been successfully achieved with a maximum Te concentration of 9 x 10(20) cm(-3). Tellurium incorporation resulted in n-doping of GaN1-xAsx layers with Hall carrier concentrations up to 3 x 10(19) cm(-3) and mobilities of similar to 1 cm(2)/V s. The optimal growth temperature window for efficient Te doping of the amorphous GaNi1-xAsx layers has been determined.

The elemental composition, structural, optical and electronic properties of p-type Cu3BiS3 thin films are investigated. The films are shown to be single phase orthorhombic, with a measured composition of Cu3.00Bi0.92S3.02. A surface oxidation layer is also clarified using energy dependent X-ray microanalysis. Photoreflectance spectra demonstrate two band gaps (EgX =1.24 eV and EgY =1.53 eV at 4 K) associated with the X and Y valence sub-bands. The photocurrent excitation measurements suggest a direct allowed nature of EgX. Photoluminescence spectra at 5 K reveal two broad emission bands at 0.84 and 0.99 eV quenching with an activation energy of 40 meV.

We report on our multi?pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3{\ensuremath{\mu}}m)/AlN(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X?ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson?Schrödinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2?DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance.

@Article{strathprints50638,
author = {G. Naresh-Kumar and A. Vilalta-Clemente and S. Pandey and D. Skuridina and H. Behmenburg and P. Gamarra and G. Patriarche and I. Vickridge and M. A. di Forte-Poisson and P. Vogt and M. Kneissl and M. Morales and P. Ruterana and A. Cavallini and D. Cavalcoli and C. Giesen and M. Heuken and C. Trager-Cowan},
title = {Multicharacterization approach for studying InAl(Ga)N/Al(Ga)N/GaN heterostructures for high electron mobility transistors},
journal = {AIP Advances},
year = {2014},
volume = {4},
number = {12},
pages = {127101},
month = {December},
abstract = {We report on our multi?pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3{\ensuremath{\mu}}m)/AlN(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X?ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson?Schr{\"o}dinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2?DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance.},
keywords = {Ga incorporation, III-V semiconductors, Rutherford backscattering, Physics, Electronic, Optical and Magnetic Materials, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/50638/}
}

We use objective boundary conditions and self-consistent charge density-functional-based tight-binding to simulate at the atomistic scale the formation of helices in narrow graphene nanoribbons with armchair edges terminated with fluorine and hydrogen. We interpret the microscopic data using an inextensible, unshearable elastic rod model, which considers both bending and torsional strains. When fitted to the atomistic data, the simple rod model uses closed-form solutions for a cubic equation to predict the strain energy and morphology at a given twist angle and the crossover point between pure torsion and a helix. Our modeling and simulation bring key insights into the origin of the helical graphene morphologies stored inside of carbon nanotubes. They can be useful for designing chiral nanoribbons with tailored properties.

@Article{strathprints50426,
author = {Ilia Nikiforov and Benjamin Hourahine and Thomas Frauenheim and Traian Dumitric{\u a}},
title = {Formation of helices in graphene nanoribbons under torsion},
journal = {Journal of Physical Chemistry Letters},
year = {2014},
volume = {5},
number = {23},
pages = {4083--4087},
month = {December},
abstract = {We use objective boundary conditions and self-consistent charge density-functional-based tight-binding to simulate at the atomistic scale the formation of helices in narrow graphene nanoribbons with armchair edges terminated with fluorine and hydrogen. We interpret the microscopic data using an inextensible, unshearable elastic rod model, which considers both bending and torsional strains. When fitted to the atomistic data, the simple rod model uses closed-form solutions for a cubic equation to predict the strain energy and morphology at a given twist angle and the crossover point between pure torsion and a helix. Our modeling and simulation bring key insights into the origin of the helical graphene morphologies stored inside of carbon nanotubes. They can be useful for designing chiral nanoribbons with tailored properties.},
keywords = {objective boundary conditions, graphene nanoribbons, strain energy, morphology, Physics, Materials Science(all)},
url = {http://strathprints.strath.ac.uk/50426/}
}

Combining a yellow-emitting organic material with a blue-emitting inorganic light-emitting device (LED) provides a hybrid white-light LED. This approach, demonstrated by R. W. Martin, P. J. Skabara, and co-workers on page 7290, couples very efficient blue emission with the flexibility, fast modulation speed, and cost effectiveness of the organic material. For the latter, BODIPY is used as the emitter and linked to a conjugated unit designed to efficiently absorb a suitable fraction of the blue light.

In this work the compositional and optical characterization of three series of dilute-Sb GaN1 ? xSbx alloys grown with various Sb flux, under N and Ga-rich conditions, are presented. Using wavelength dispersive x-ray microanalysis and Rutherford backscattering spectroscopy it is found that the N-rich samples (Ga flux {\ensuremath{<}} 2.3 {$\times$} 10?7 Torr) incorporate a higher magnitude of GaSb than the Ga-rich samples (Ga flux {\ensuremath{>}} 2.3 {$\times$} 10?7 Torr) under the same growth conditions. The optical properties of the Ga-rich samples are measured using room temperature cathodoluminescence (CL), photoluminescence (PL) and absorption measurements. A broad luminescence peak is observed around 2.2 eV. The nature and properties of this peak are considered, as is the suitability of these dilute-Sb alloys for use in solar energy conversion devices.

Cathodoluminescence (CL) hyperspectral imaging has been performed on GaN nanorods containing a single InGaN quantum disk (SQD) with controlled variations in excitation conditions. Two different nanorod diameters (200 and 280 nm) have been considered. Systematic changes in the CL spectra from the SQD were observed as the accelerating voltage of the electron beam and its position of incidence are varied. It is shown that the dominant optical transition in the SQD varies across the nanorod as a result of interplay between the contributions of the deformation potential and the quantum-confined Stark effect to the transition energy as consequence of radial variation in the pseudomorphic strain.

@Article{strathprints50121,
author = {Y. D. Zhuang and J. Bruckbauer and P. A. Shields and P. R. Edwards and R. W. Martin and D. W. E. Allsopp},
title = {Influence of stress on optical transitions in {GaN} nanorods containing a single {InGaN/GaN} quantum disk},
journal = {Journal of Applied Physics},
year = {2014},
volume = {116},
number = {17},
pages = {174305},
month = {November},
abstract = {Cathodoluminescence (CL) hyperspectral imaging has been performed on GaN nanorods containing a single InGaN quantum disk (SQD) with controlled variations in excitation conditions. Two different nanorod diameters (200 and 280 nm) have been considered. Systematic changes in the CL spectra from the SQD were observed as the accelerating voltage of the electron beam and its position of incidence are varied. It is shown that the dominant optical transition in the SQD varies across the nanorod as a result of interplay between the contributions of the deformation potential and the quantum-confined Stark effect to the transition energy as consequence of radial variation in the pseudomorphic strain.},
keywords = {nanorods, electron beams, emission spectra, quantum wells, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/50121/}
}

InAlN epilayers deposited on thick GaN buffer layers grown by metalorganic chemical vapour deposition (MOCVD) revealed an auto-incorporation of Ga when analysed by wavelength dispersive x-ray (WDX) spectroscopy and Rutherford backscattering spectrometry (RBS). Samples were grown under similar conditions with the change in reactor flow rate resulting in varying Ga contents of 12-24\%. The increase in flow rate from 8000 to 24 000 sccm suppressed the Ga auto-incorporation which suggests that the likely cause is from residual Ga left behind from previous growth runs. The luminescence properties of the resultant InAlGaN layers were investigated using cathodoluminescence (CL) measurements.

Cathodoluminescence (CL) is the electron-stimulated emission of low-energy (IR/visible/UV) photons from a solid material. Electron irradiation raises sample electrons to an excited state, which then emit a photon as they return to a lower energy state. The resultant luminescence can be analysed both spatially and spectrally, and until recently only one of these two approaches could be used for a given measurement. This chapter outlines the conventional spatial and spectral techniques, then describes the more recent approach of hyperspectral imaging, in which a single CL dataset simultaneously contains both spatial and spectral information.

@incollection{strathprints49411,
author = {Paul R. Edwards and Martin R. Lee},
series = {Short Course Series},
booktitle = {Cathodoluminescence and its Application to Geoscience},
editor = {Ian M. Coulson},
address = {Qu{\'e}bec},
title = {Cathodoluminescence hyperspectral imaging in geoscience},
publisher = {Mineralogical Association of Canada},
year = {2014},
pages = {29--45},
keywords = {cathodoluminescence, CL, hyperspectral imaging (HSI), Physics, Geology, Physics and Astronomy (miscellaneous), Geophysics},
url = {http://strathprints.strath.ac.uk/49411/},
abstract = {Cathodoluminescence (CL) is the electron-stimulated emission of low-energy (IR/visible/UV) photons from a solid material. Electron irradiation raises sample electrons to an excited state, which then emit a photon as they return to a lower energy state. The resultant luminescence can be analysed both spatially and spectrally, and until recently only one of these two approaches could be used for a given measurement. This chapter outlines the conventional spatial and spectral techniques, then describes the more recent approach of hyperspectral imaging, in which a single CL dataset simultaneously contains both spatial and spectral information.}
}

AlGaN/AlGaN multi-quantum-wells (MQW) with AlN-rich grains have been grown by metal organic chemical vapor deposition. The grains are observed to have strong excitonic localization characteristics that are affected by their sizes. The tendency to confine excitons progressively intensifies with increasing grain boundary area. Photoluminescence results indicate that the MQW have a dominant effect on the peak energy of the near-bandedge emission at temperatures below 150 K, with the localization properties of the grains becoming evident beyond 150 K. Cathodoluminescence maps reveal that the grain boundary has no effect on the peak intensities of the AlGaN/AlGaN samples.

@Article{strathprints49377,
author = {Idris A. Ajia and P. R. Edwards and Z. Liu and J. C. Yan and R. W. Martin and I. S. Roqan},
title = {Excitonic localization in {AlN}-rich {AlxGa1-xN/AlyGa1-yN} multi-quantum-well grain boundaries},
journal = {Applied Physics Letters},
year = {2014},
volume = {105},
number = {12},
pages = {122111},
month = {September},
note = {. Copyright (2014) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.},
abstract = {AlGaN/AlGaN multi-quantum-wells (MQW) with AlN-rich grains have been grown by metal organic chemical vapor deposition. The grains are observed to have strong excitonic localization characteristics that are affected by their sizes. The tendency to confine excitons progressively intensifies with increasing grain boundary area. Photoluminescence results indicate that the MQW have a dominant effect on the peak energy of the near-bandedge emission at temperatures below 150 K, with the localization properties of the grains becoming evident beyond 150 K. Cathodoluminescence maps reveal that the grain boundary has no effect on the peak intensities of the AlGaN/AlGaN samples.},
keywords = {multi-quantum-wells, MQW, AlN-rich grains, excitonic localization, AlGaN/AlGaN MQW, Physics, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/49377/}
}

We show that arrays of emissive nanorod structures can exhibit strong photonic crystal behavior, via observations of the far-field luminescence from core-shell and quantum disc InGaN/GaN nanorods. The conditions needed for the formation of directional Bloch modes characteristic of strong photonic behavior are found to depend critically upon the vertical shape of the nanorod sidewalls. Index guiding by a region of lower volume-averaged refractive index near the base of the nanorods creates a quasi-suspended photonic crystal slab at the top of the nanorods which supports Bloch modes. Only diffractive behavior could be observed without this region. Slab waveguide modelling of the vertical structure shows that the behavioral regime of the emissive nanorod arrays depends strongly upon the optical coupling between the nanorod region and the planar layers below. The controlled crossover between the two regimes of photonic crystal operation enables the design of photonic nanorod structures formed on planar substrates that exploit either behavior depending on device requirements.

@Article{strathprints49269,
author = {C. J. Lewins and E. D. Le Boulbar and S. M. Lis and P. R. Edwards and R. W. Martin and P. A. Shields and D. W. E. Allsopp},
title = {Strong photonic crystal behavior in regular arrays of core-shell and quantum disc InGaN/GaN nanorod light-emitting diodes},
journal = {Journal of Applied Physics},
year = {2014},
volume = {116},
number = {4},
pages = {044305},
abstract = {We show that arrays of emissive nanorod structures can exhibit strong photonic crystal behavior, via observations of the far-field luminescence from core-shell and quantum disc InGaN/GaN nanorods. The conditions needed for the formation of directional Bloch modes characteristic of strong photonic behavior are found to depend critically upon the vertical shape of the nanorod sidewalls. Index guiding by a region of lower volume-averaged refractive index near the base of the nanorods creates a quasi-suspended photonic crystal slab at the top of the nanorods which supports Bloch modes. Only diffractive behavior could be observed without this region. Slab waveguide modelling of the vertical structure shows that the behavioral regime of the emissive nanorod arrays depends strongly upon the optical coupling between the nanorod region and the planar layers below. The controlled crossover between the two regimes of photonic crystal operation enables the design of photonic nanorod structures formed on planar substrates that exploit either behavior depending on device requirements.},
keywords = {nanorods, photonic crystals, light emitting diodes , III-V semiconductors, light diffraction, Solid state physics. Nanoscience, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/49269/}
}

In this work, we demonstrate the use of hyperspectral CL in the evaluation of periodic arrays of GaN/InxGa1-xN core-shell nanorods. These were fabricated using a top-down approach, in which columns are formed from a GaN template using nano-imprint lithography and ICP etching, followed by MOCVD regrowth [2]. The formation of quantum wells (QWs) on the mplane sidewall facets offers a route to avoiding the detrimental electric fields associated with LEDs grown on the c-plane, while the use of periodic features has the potential to improve light extraction and directionality.

@inproceedings{strathprints49238,
booktitle = {Condensed Matter in Paris 2014 (CMD25-JMC14)},
title = {Cathodoluminescence hyperspectral imaging of nitride core-shell structures},
author = {P. R. Edwards and E. D. Le Boulbar and P. A. Shields and D. W. E. Allsopp and R. W. Martin},
year = {2014},
pages = {679--680},
journal = {Condensed Matter in Paris 2014 (CMD25-JMC14)},
keywords = {spectroscopic techniques, cathodoluminescence (CL), hyperspectral imaging (HSI), nitride nanostructures, Physics, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/49238/},
abstract = {In this work, we demonstrate the use of hyperspectral CL in the evaluation of periodic arrays of GaN/InxGa1-xN core-shell nanorods. These were fabricated using a top-down approach, in which columns are formed from a GaN template using nano-imprint lithography and ICP etching, followed by MOCVD regrowth [2]. The formation of quantum wells (QWs) on the mplane sidewall facets offers a route to avoiding the detrimental electric fields associated with LEDs grown on the c-plane, while the use of periodic features has the potential to improve light extraction and directionality.}
}

A novel BODIPY-containing organic small molecule is synthesized and employed as a down-converting layer on a commercial blue light-emitting diode (LED). The resulting hybrid device demonstrates white light emission under low-current operation, with color coordinates of (0.34, 0.31) and an efficacy of 13.6 lm/W; four times greater than the parent blue LED.

Micron-scale mapping has been employed to study a contacted InGaN/GaN LED using combined electroluminescence (EL), cathodoluminescence (CL), and electron beam induced current (EBIC). Correlations between parameters, such as the EBIC and CL intensity, were studied as a function of applied bias. The CL and EBIC maps reveal small areas, 2?10 {\ensuremath{\mu}}m in size, which have increased nonradiative recombination rate and/or a lower conductivity. The CL emission from these spots is blue shifted, by 30?40 meV. Increasing the reverse bias causes the size of the spots to decrease, due to competition between in-plane diffusion and drift in the growth direction. EL mapping shows large bright areas ({$\sim$}100 {\ensuremath{\mu}}m) which also have increased EBIC, indicating domains of increased conductivity in the p and/or n-GaN.

Abstract Thin films of p-type Cu3BiS3 with an orthorhombic wittichenite structure, a semiconductor with high potential for thin film solar cell absorber layers, were synthesised by thermal annealing of Cu and Bi precursors, magnetron sputtered on Mo/glass substrate, with a layer of thermo-evaporated S. The elemental composition, structural and electronic properties are studied. The Raman spectrum shows four modes with the dominant peak at 292 cm-1. Photoreflectance spectra demonstrate two band gaps EgX and EgY, associated with the X and Y valence sub-bands, and their evolution with temperature. Fitting the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for EgX and EgY at 0 K as well as the average phonon energy. Photoluminescence spectra at 5 K reveal two bright and broad emission bands at 0.84 and 0.99 eV, which quench with an activation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse and suggest a direct allowed nature of the band gap.

@article{strathprints48646,
volume = {562},
title = {Electronic and structural characterisation of Cu3BiS3 thin films for the absorber layer of sustainable photovoltaics},
author = {M.V. Yakushev and P. Maiello and T. Raadik and M.J. Shaw and P.R. Edwards and J. Krustok and A.V. Mudryi and I. Forbes and R.W. Martin},
year = {2014},
pages = {195--199},
note = {{\copyright} 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license},
journal = {Thin Solid Films},
keywords = {photoluminescence, thin films, solar cells, semiconductors, electronic structure, raman spectroscopy, photoreflectance, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/48646/},
abstract = {Abstract Thin films of p-type Cu3BiS3 with an orthorhombic wittichenite structure, a semiconductor with high potential for thin film solar cell absorber layers, were synthesised by thermal annealing of Cu and Bi precursors, magnetron sputtered on Mo/glass substrate, with a layer of thermo-evaporated S. The elemental composition, structural and electronic properties are studied. The Raman spectrum shows four modes with the dominant peak at 292 cm-1. Photoreflectance spectra demonstrate two band gaps EgX and EgY, associated with the X and Y valence sub-bands, and their evolution with temperature. Fitting the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for EgX and EgY at 0 K as well as the average phonon energy. Photoluminescence spectra at 5 K reveal two bright and broad emission bands at 0.84 and 0.99 eV, which quench with an activation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse and suggest a direct allowed nature of the band gap.}
}

Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundamental modes inside and outside of the disc were performed. Simulated images reproduced the observed spatial structures excited in the disc. Mode-analysis of the simulated images indicates that the spatial features observed in the transmission images originate mainly from a few fundamental plasmon modes of the disc.

@article{strathprints48128,
volume = {22},
number = {10},
title = {Plasmon modes in single gold nanodiscs},
author = {Kohei Imura and Kosei Ueno and Hiroaki Misawa and Hiromi Okamoto and Duncan McArthur and Benjamin Hourahine and Francesco Papoff},
year = {2014},
pages = {12189--12199},
journal = {Optics Express},
keywords = {optical properties, single gold nanodiscs, plasmon modes, near-field optical microscopy, Physics, Solid state physics. Nanoscience, Optics. Light, Physics and Astronomy(all), Atomic and Molecular Physics, and Optics},
url = {http://strathprints.strath.ac.uk/48128/},
abstract = {Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundamental modes inside and outside of the disc were performed. Simulated images reproduced the observed spatial structures excited in the disc. Mode-analysis of the simulated images indicates that the spatial features observed in the transmission images originate mainly from a few fundamental plasmon modes of the disc.}
}

A method to grow high quality, single crystalline semiconductor material irrespective of the substrate would allow a cost-effective improvement to functionality and performance of optoelectronic devices. Recently, a novel type of substrate-insensitive growth process called Evolutionary Selection Selective Area Growth (ES-SAG) has been proposed. Here we report the use of X-ray microdiffraction to study the structural properties of GaN microcrystals grown by ES-SAG. Utilizing high resolution in both direct and reciprocal spaces, we have unraveled structural dynamics of GaN microcrystals in growth structures of different dimensions. It has been found that the geometric proportions of the growth constrictions play an important role: 2.6â.Î 1/4m and 4.5â.Î 1/4m wide growth tunnels favor the evolutionary selection mechanism, contrary to the case of 8.6â.Î 1/4m growth tunnels. It was also found that GaN microcrystal ensembles are dominated by slight tensile strain irrespective of growth tunnel shape.

@Article{strathprints47832,
author = {Vyacheslav V. Kachkanov and Benjamin B. Leung and Jie J. Song and Jung J. Han and Y. Zhang and M.C. Tsai and G. Yuan and J. Han and Kevin K. O'Donnell},
title = {Structural dynamics of GaN microcrystals in evolutionary selection selective area growth probed by {X}-ray microdiffraction},
journal = {Scientific Reports},
year = {2014},
volume = {4},
pages = {4651},
month = {April},
abstract = {A method to grow high quality, single crystalline semiconductor material irrespective of the substrate would allow a cost-effective improvement to functionality and performance of optoelectronic devices. Recently, a novel type of substrate-insensitive growth process called Evolutionary Selection Selective Area Growth (ES-SAG) has been proposed. Here we report the use of X-ray microdiffraction to study the structural properties of GaN microcrystals grown by ES-SAG. Utilizing high resolution in both direct and reciprocal spaces, we have unraveled structural dynamics of GaN microcrystals in growth structures of different dimensions. It has been found that the geometric proportions of the growth constrictions play an important role: 2.6{\^a}.{\^I} 1/4m and 4.5{\^a}.{\^I} 1/4m wide growth tunnels favor the evolutionary selection mechanism, contrary to the case of 8.6{\^a}.{\^I} 1/4m growth tunnels. It was also found that GaN microcrystal ensembles are dominated by slight tensile strain irrespective of growth tunnel shape.},
keywords = {electronic devices, structure of solids, structure of liquids, GaN microcrystals, Physics, General},
url = {http://strathprints.strath.ac.uk/47832/}
}

Rare-earth doped III-N semiconductors have been studied for decades on account of their possible application in visible light-emitting diodes (LED) with built-in utility as red (e.g. Eu), green (Er) and blue (Tm) monochromatic sources (O’Donnell and Dierolf (eds.), Topics in Applied Physics, Vol. 124 (Springer, Dordrecht, 2010) [1]). However, to date, no commercial devices have been introduced on the basis of these materials. Recently, we discovered thermally activated hysteresis in the emission spectrum of p-type GaN thin films that were co-doped with Mg and Eu (O’Donnell et al., Proc. ICPS31, Zurich, July 2012 [2]). We have also reported an unexpected Zeeman splitting and induced magnetic moment of Eu3+ ions in GaN (Kachkanov et al., Scientific Rep. 2, 969 (2012) and MRS Proc. 1290?i03?06 (2011) [3, 4]). These findings encourage speculation on taking the study of RE-doped III-N beyond the limited goal of improving LED efficiency into the realm of novel magneto-optic and quantum-optical devices. In particular we will describe in this presentation the spectroscopy of ion-implanted and annealed GaN(Mg): Eu samples and the possible exploitation of the Mg acceptor in GaN as a qubit.

Cu(In,Ga)Se (CIGS) thin films were deposited by a two-step process on Mo-coated soda-lime glass substrates. The CuInGa (CIG) precursors were prepared in an in-line evaporation system at room temperature, and then selenised at 500 ?C. The two-step processed CIGS films were mechanically compressed at 25 MPa to improve their optoelectronic properties, which were verified by photoluminescence (PL). The surface and structural properties were compared before and after compression. The mechanical compression has brought changes in the surface morphology and porosity without changing the structural properties of the material. The PL technique has been used to reveal changes in the electronic properties of the films. PL spectra at different excitation laser powers and temperatures were measured for as-grown as well as compressed samples. The PL spectra of the as-grown films revealed three broad and intense bands shifting at a significant rate towards higher energies (j-shift) with the increase in excitation power suggesting that the material is highly doped and compensated. At increasing temperature, the bands shift towards lower energies, which is a characteristic of the band tails generated by spatial potential fluctuation. The compression increases the intensity of energy bands by an order of magnitude and reduces the j-shift, demonstrating an improvement of the electronic properties. {\copyright} 2014 The Royal Society of Chemistry.

@article{strathprints47229,
volume = {4},
number = {10},
month = {January},
author = {Z. Wei and S. Senthilarasu and M.V. Yakushev and R.W. Martin and H.M. Upadhyaya},
title = {Effect of mechanical compression on Cu(In,Ga)Se films : micro-structural and photoluminescence analysis},
journal = {RSC Advances},
pages = {5141--5147},
year = {2014},
keywords = {mechanical compression, Cu(In,Ga)Se films, micro-structural , photoluminescence analysis, Chemical engineering, Solid state physics. Nanoscience, Chemical Engineering(all), Chemistry(all)},
url = {http://strathprints.strath.ac.uk/47229/},
abstract = {Cu(In,Ga)Se (CIGS) thin films were deposited by a two-step process on Mo-coated soda-lime glass substrates. The CuInGa (CIG) precursors were prepared in an in-line evaporation system at room temperature, and then selenised at 500 ?C. The two-step processed CIGS films were mechanically compressed at 25 MPa to improve their optoelectronic properties, which were verified by photoluminescence (PL). The surface and structural properties were compared before and after compression. The mechanical compression has brought changes in the surface morphology and porosity without changing the structural properties of the material. The PL technique has been used to reveal changes in the electronic properties of the films. PL spectra at different excitation laser powers and temperatures were measured for as-grown as well as compressed samples. The PL spectra of the as-grown films revealed three broad and intense bands shifting at a significant rate towards higher energies (j-shift) with the increase in excitation power suggesting that the material is highly doped and compensated. At increasing temperature, the bands shift towards lower energies, which is a characteristic of the band tails generated by spatial potential fluctuation. The compression increases the intensity of energy bands by an order of magnitude and reduces the j-shift, demonstrating an improvement of the electronic properties. {\copyright} 2014 The Royal Society of Chemistry.}
}

Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (90 meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (10 meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40\% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes.

@Article{strathprints47167,
author = {Jochen Bruckbauer and Paul R Edwards and Suman-Lata Sahonta and Fabien C-P Massabuau and Menno J Kappers and Colin J Humphreys and Rachel A Oliver and Robert W Martin},
title = {Cathodoluminescence hyperspectral imaging of trench-like defects in {InGaN/GaN} quantum well structures},
journal = {Journal of Physics D: Applied Physics},
year = {2014},
volume = {47},
number = {13},
pages = {135107},
month = {March},
abstract = {Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (90 meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (10 meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40\% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes.},
keywords = {cathodoluminescence , hyperspectral imaging, trench-like defects, nGaN/GaN , quantum well structures, Physics, Solid state physics. Nanoscience, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/47167/}
}

Sequential multiple Eu ion implantations at low fluence (1{$\times$}1013 cm-2 at 300 keV) and subsequent rapid thermal annealing (RTA) steps (30 s at 1000 ?C or 1100 ?C) were performed on high quality nominally undoped GaN films grown by metal organic chemical vapour deposition (MOCVD) and medium quality GaN:Mg grown by hydride vapour phase epitaxy (HVPE). Compared to samples implanted in a single step, multiple implantation/annealing shows only marginal structural improvement for the MOCVD samples, but a significant improvement of crystal quality and optical activation of Eu was achieved in the HVPE films. This improvement is attributed to the lower crystalline quality of the starting material, which probably enhances the diffusion of defects and acts to facilitate the annealing of implantation damage and the effective incorporation of the Eu ions in the crystal structure. Optical activation of Eu3+ ions in the HVPE samples was further improved by high temperature and high pressure annealing (HTHP) up to 1400 ?C. After HTHP annealing the main room temperature cathodo- and photoluminescence line in Mg-doped samples lies at {$\sim$} 619 nm, characteristic of a known Mg-related Eu3+ centre, while after RTA treatment the dominant line lies at {$\sim$} 622 nm, typical for undoped GaN:Eu.

@Article{strathprints47160,
author = {S. M. C. Miranda and P. R. Edwards and K. P. O'Donnell and M. Bo{\'c}kowski and E. Alves and I. S. Roqan and A. Vantomme and K. Lorenz},
title = {Sequential multiple-step europium ion implantation and annealing of {GaN}},
journal = {Physica Status Solidi C},
year = {2014},
volume = {11},
number = {2},
pages = {253--257},
month = {February},
abstract = {Sequential multiple Eu ion implantations at low fluence (1{$\times$}1013 cm-2 at 300 keV) and subsequent rapid thermal annealing (RTA) steps (30 s at 1000 ?C or 1100 ?C) were performed on high quality nominally undoped GaN films grown by metal organic chemical vapour deposition (MOCVD) and medium quality GaN:Mg grown by hydride vapour phase epitaxy (HVPE). Compared to samples implanted in a single step, multiple implantation/annealing shows only marginal structural improvement for the MOCVD samples, but a significant improvement of crystal quality and optical activation of Eu was achieved in the HVPE films. This improvement is attributed to the lower crystalline quality of the starting material, which probably enhances the diffusion of defects and acts to facilitate the annealing of implantation damage and the effective incorporation of the Eu ions in the crystal structure. Optical activation of Eu3+ ions in the HVPE samples was further improved by high temperature and high pressure annealing (HTHP) up to 1400 ?C. After HTHP annealing the main room temperature cathodo- and photoluminescence line in Mg-doped samples lies at {$\sim$} 619 nm, characteristic of a known Mg-related Eu3+ centre, while after RTA treatment the dominant line lies at {$\sim$} 622 nm, typical for undoped GaN:Eu.},
keywords = {crystal quality, europium, gallium nitride, ion implantation, multiple-step , GaN, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/47160/}
}

The influence of substrate miscut on Al0.5Ga0.5 N layers was investigated using cathodoluminescence (CL) hyperspectral imaging and secondary electron imaging in an environmental scanning electron microscope. The samples were also characterized using atomic force microscopy and high resolution X-ray diffraction. It was found that small changes in substrate miscut have a strong influence on the morphology and luminescence properties of the AlGaN layers. Two different types are resolved. For low miscut angle, a crack-free morphology consisting of randomly sized domains is observed, between which there are notable shifts in the AlGaN near band edge emission energy. For high miscut angle, a morphology with step bunches and compositional inhomogeneities along the step bunches, evidenced by an additional CL peak along the step bunches, are observed.

@Article{strathprints47137,
author = {Gunnar Kusch and Haoning Li and Paul R. Edwards and Jochen Bruckbauer and Thomas C. Sadler and Peter J. Parbrook and Robert W. Martin},
title = {Influence of substrate miscut angle on surface morphology and luminescence properties of AlGaN},
journal = {Applied Physics Letters},
year = {2014},
volume = {104},
number = {9},
pages = {092114},
month = {March},
abstract = {The influence of substrate miscut on Al0.5Ga0.5 N layers was investigated using cathodoluminescence (CL) hyperspectral imaging and secondary electron imaging in an environmental scanning electron microscope. The samples were also characterized using atomic force microscopy and high resolution X-ray diffraction. It was found that small changes in substrate miscut have a strong influence on the morphology and luminescence properties of the AlGaN layers. Two different types are resolved. For low miscut angle, a crack-free morphology consisting of randomly sized domains is observed, between which there are notable shifts in the AlGaN near band edge emission energy. For high miscut angle, a morphology with step bunches and compositional inhomogeneities along the step bunches, evidenced by an additional CL peak along the step bunches, are observed.},
keywords = {cathodoluminescence, III-V semiconductors, surface morphology, X-ray diffraction, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/47137/}
}

We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black?white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.

@article{strathprints46655,
volume = {20},
number = {1},
month = {February},
author = {G. Naresh-Kumar and J. Bruckbauer and P. R. Edwards and S. Kraeusel and B. Hourahine and R. W. Martin and M. J. Kappers and M. A. Moram and S. Lovelock and R. A. Oliver and C. J. Humphreys and C. Trager-Cowan},
title = {Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN},
journal = {Microscopy and Microanalysis},
pages = {55--60},
year = {2014},
keywords = {electron channeling, cathodoluminescence studies, threading, dislocations, GaN, Instrumentation},
url = {http://strathprints.strath.ac.uk/46655/},
abstract = {We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black?white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.}
}

We compare two ways of calculating the optical response of metallic nanoparticles illuminated by near field dipole sources. We develop tests to determine the accuracy of the calculations of internal and scattered fields of metallic nanoparticles at the boundary of the particles and in the far field. We verify the correct transport of energy by checking that the evaluation of the energy flux agrees at the surface of the particles and in the far field. A new test is introduced to check that the surface fields fulfill Maxwell’s equations allowing evaluation of the validity of the internal field. Calculations of the scattering cross section show a faster rate of convergence for the principal mode theory. We show that for metallic particles the internal field is the most significant source of error.

@article{strathprints46439,
volume = {4},
number = {4},
month = {May},
author = {Duncan McArthur and Benjamin Hourahine and Francesco Papoff},
title = {Evaluation of E. M. fields and energy transport in metallic nanoparticles with near field excitation},
journal = {Physical Science International Journal},
pages = {564--575},
year = {2014},
keywords = {nanoparticles, electromagnetic scattering, energy flux, nanophotonics, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/46439/},
abstract = {We compare two ways of calculating the optical response of metallic nanoparticles illuminated by near field dipole sources. We develop tests to determine the accuracy of the calculations of internal and scattered fields of metallic nanoparticles at the boundary of the particles and in the far field. We verify the correct transport of energy by checking that the evaluation of the energy flux agrees at the surface of the particles and in the far field. A new test is introduced to check that the surface fields fulfill Maxwell's equations allowing evaluation of the validity of the internal field. Calculations of the scattering cross section show a faster rate of convergence for the principal mode theory. We show that for metallic particles the internal field is the most significant source of error.}
}

Studies of ambient-pressure and high-pressure behavior of photoluminescence (PL) for series of InxAl1-xN layers are presented. The measured evolution of PL energy (EPL) with x is characterized by a clear decrease of EPL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band-gap energy changes EG with x. However, values of EPL are clearly lower than EG, for 0{\ensuremath{<}}x{\ensuremath{<}}0.3. For higher x, the measured EPL follows well the calculated EG. The experimentally determined pressure coefficient of PL energy (dEPL/dp) shows a complicated behavior for alloys with different In-content. We found a strong reduction of dEPL/dp for 0{\ensuremath{<}}x{\ensuremath{<}}0.3 and a relatively constant magnitude of this coefficient for higher x. Moreover, for the lower x region, we observed dEPL/dp that can differ even by a factor two in samples with nominally very similar In-content. The general tendency in dEPL/dp evolution with x corresponds to lower values than calculated dEG/dp for alloys with non-uniform indium distribution. We propose two not necessary independent explanations of these experimental findings. First, due to non-uniform In distribution (induced, e.g. by defects or non-homogeneous strain) both EPL and dEPL/dp are reduced. Second, a similar behavior results from an involvement of the localized states, whose formation and contribution to PL can be induced by strain and/or native defects. In both hypotheses, the strain/defect density can significantly change around x{$\approx$}0.18 where InxAl1-xN layers are lattice matched to GaN template. {\copyright} 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

@article{strathprints48001,
volume = {250},
number = {4},
month = {April},
author = {Agata Kaminska and Piotr Nowakowski and Grzegorz Staszczak and Tadeusz Suski and Andrzej Suchocki and Jean Fran{\c c}ois Carlin and Nicolas Grandjean and Robert Martin and Akio Yamamoto},
title = {Peculiarities in the pressure dependence of photoluminescence in InAlN},
journal = {Physica Status Solidi B},
pages = {677--682},
year = {2013},
keywords = {high-pressure spectroscopy, photoluminescence, pressure coefficients, ternary nitride alloys, InAlN, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials},
url = {http://strathprints.strath.ac.uk/48001/},
abstract = {Studies of ambient-pressure and high-pressure behavior of photoluminescence (PL) for series of InxAl1-xN layers are presented. The measured evolution of PL energy (EPL) with x is characterized by a clear decrease of EPL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band-gap energy changes EG with x. However, values of EPL are clearly lower than EG, for 0{\ensuremath{<}}x{\ensuremath{<}}0.3. For higher x, the measured EPL follows well the calculated EG. The experimentally determined pressure coefficient of PL energy (dEPL/dp) shows a complicated behavior for alloys with different In-content. We found a strong reduction of dEPL/dp for 0{\ensuremath{<}}x{\ensuremath{<}}0.3 and a relatively constant magnitude of this coefficient for higher x. Moreover, for the lower x region, we observed dEPL/dp that can differ even by a factor two in samples with nominally very similar In-content. The general tendency in dEPL/dp evolution with x corresponds to lower values than calculated dEG/dp for alloys with non-uniform indium distribution. We propose two not necessary independent explanations of these experimental findings. First, due to non-uniform In distribution (induced, e.g. by defects or non-homogeneous strain) both EPL and dEPL/dp are reduced. Second, a similar behavior results from an involvement of the localized states, whose formation and contribution to PL can be induced by strain and/or native defects. In both hypotheses, the strain/defect density can significantly change around x{$\approx$}0.18 where InxAl1-xN layers are lattice matched to GaN template. {\copyright} 2013 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim.}
}

A combined experimental and theoretical study of the band gap of AllnN is presented, which confirms the breakdown of the virtual crystal approximation (VCA) for the conduction and valence band edges. Composition-dependent bowing parameters for these quantities are extracted. Additionally, composition-dependent band offsets for GaN/AllnN systems are provided. We show that local strain and built-in fields affect the band edges significantly, leading to optical polarization switching at a much lower In composition than expected from a VCA approach.

@Article{strathprints47231,
author = {S. Schulz and M.A. Caro and L.-T. Tan and P.J. Parbrook and R.W. Martin and E.P. O'Reilly},
title = {Composition-dependent band gap and band-edge bowing in AIInN : a combined theoretical and experimental study},
journal = {Applied Physics Express},
year = {2013},
volume = {6},
number = {12},
pages = {121001},
month = {December},
abstract = {A combined experimental and theoretical study of the band gap of AllnN is presented, which confirms the breakdown of the virtual crystal approximation (VCA) for the conduction and valence band edges. Composition-dependent bowing parameters for these quantities are extracted. Additionally, composition-dependent band offsets for GaN/AllnN systems are provided. We show that local strain and built-in fields affect the band edges significantly, leading to optical polarization switching at a much lower In composition than expected from a VCA approach.},
keywords = {composition-dependent, band gap, band-edge, bowing, AlInN, experimental study, Physics, Physics and Astronomy(all), Engineering(all)},
url = {http://strathprints.strath.ac.uk/47231/}
}

GaN materials alloyed with group V anions form the so-called highly mismatched alloys (HMAs). Recently, the authors succeeded in growing N-rich GaNAs and GaNBi alloys over a large composition range by plasma-assisted molecular beam epitaxy (PA-MBE). Here, they present first results on PA-MBE growth and properties of N-rich GaNSb and InNAs alloys and compare these with GaNAs and GaNBi alloys. The enhanced incorporation of As and Sb was achieved by growing the layers at extremely low growth temperatures. Although layers become amorphous for high As, Sb, and Bi content, optical absorption measurements show a progressive shift of the optical absorption edge to lower energy. The large band gap range and controllable conduction and valence band positions of these HMAs make them promising materials for efficient solar energy conversion devices.

A red emission site (hereafter, Eu0), with its main 5D0 to 7F2 peak at 619 nm, is observed by photoluminescence (PL) spectroscopy of Eu-implanted, Mg-doped GaN, in samples annealed at high temperature and pressure (up to 1400 ?C, 1 GPa) in order to remove lattice damage. The PL spectrum is strongly temperature-hysteretic between room temperature and {$\sim$}20 K: below 30 K, photochromic switching occurs between Eu0 and the usually dominant Eu1 center; upon warming the sample, the Eu0 signal does not recover until the temperature reaches {$\sim$}150 K. Photobleaching of Eu1 takes place at low temperatures after cooling, while photo-enhancement of Eu0 takes place at high temperatures after re-warming. These observations suggest a microscopic model of charge-driven defect interconversion in p-type GaN:Eu, Mg.

We explore the generalization to the helical case of the classical Ewald method, the harbinger of all modern self-consistent treatments of waves in crystals, including ab initio electronic structure methods. Ewald-like formulas that do not rely on a unit cell with translational symmetry prove to be numerically tractable and able to provide the crucial component needed for coupling objective molecular dynamics with the self-consistent charge density-functional based tight-binding treatment of the inter-atomic interactions. The robustness of the method in addressing complex hetero-nuclear nano- and bio-systems is demonstrated with illustrative simulations on a helical boron nitride nanotube, a screw dislocated zinc oxide nanowire, and an ideal DNA molecule.

The use of etched nanorods from a planar template as a growth scaffold for a highly regular GaN/InGaN/GaN core-shell structure is demonstrated. The recovery of m-plane non-polar facets from etched high-aspect-ratio GaN nanorods is studied with and without the introduction of a hydrogen silsesquioxane passivation layer at the bottom of the etched nanorod arrays. This layer successfully prevented c-plane growth between the nanorods, resulting in vertical nanorod sidewalls ({$\sim$}89.8?) and a more regular height distribution than re-growth on unpassivated nanorods. The height variation on passivated nanorods is solely determined by the uniformity of nanorod diameter, which degrades with increased growth duration. Facet-dependent indium incorporation of GaN/InGaN/GaN core-shell layers regrown onto the etched nanorods is observed by high-resolution cathodoluminescence imaging. Sharp features corresponding to diffracted wave-guide modes in angle-resolved photoluminescence measurements are evidence of the uniformity of the full core-shell structure grown on ordered etched nanorods.

We find exact conditions for the enhancement or suppression of internal and/or scattered fields in any smooth particle and the determination of their spatial distribution or angular momentum through the combination of simple fields. The incident fields can be generated by a single monochromatic or broad band light source, or by several sources, which may also be impurities embedded in the nanoparticle. We can design the lineshape of a particle introducing very narrow features in its spectral response.

@article{strathprints44539,
volume = {21},
number = {17},
month = {August},
author = {Benjamin Hourahine and Francesco Papoff},
note = {9 pages, 5 figures},
title = {Optical control of scattering, absorption and lineshape in nanoparticles},
year = {2013},
journal = {Optics Express},
pages = {20322--20333},
keywords = {physics, optics, optical control, scattering , absorption, lineshape, nanoparticles, Physics, Atomic and Molecular Physics, and Optics},
url = {http://strathprints.strath.ac.uk/44539/},
abstract = {We find exact conditions for the enhancement or suppression of internal and/or scattered fields in any smooth particle and the determination of their spatial distribution or angular momentum through the combination of simple fields. The incident fields can be generated by a single monochromatic or broad band light source, or by several sources, which may also be impurities embedded in the nanoparticle. We can design the lineshape of a particle introducing very narrow features in its spectral response.}
}

Significant improvements in the efficiency of optoelectronic devices can result from the exploitation of nanostructures. These require optimal nanocharacterization techniques to fully understand and improve their performance. In this study we employ room temperature cathodoluminescence hyperspectral imaging to probe single GaN-based nanorods containing multiple quantum wells (MQWs) with a simultaneous combination of very high spatial and spectral resolution. We have investigated the strain state and carrier transport in the vicinity of the MQWs, demonstrating the high efficiencies resulting from reduced electric fields. Power-dependent photoluminescence spectroscopy of arrays of these nanorods confirms that their fabrication results in partial strain relaxation in the MQWs. Our technique allows us to interrogate the structures on a sufficiently small length scale to be able to extract the important information.

@Article{strathprints44537,
author = {Jochen Bruckbauer and Paul R Edwards and Jie Bai and Tao Wang and Robert W Martin},
title = {Probing light emission from quantum wells within a single nanorod},
journal = {Nanotechnology},
year = {2013},
volume = {24},
number = {36},
pages = {365704},
month = {August},
abstract = {Significant improvements in the efficiency of optoelectronic devices can result from the exploitation of nanostructures. These require optimal nanocharacterization techniques to fully understand and improve their performance. In this study we employ room temperature cathodoluminescence hyperspectral imaging to probe single GaN-based nanorods containing multiple quantum wells (MQWs) with a simultaneous combination of very high spatial and spectral resolution. We have investigated the strain state and carrier transport in the vicinity of the MQWs, demonstrating the high efficiencies resulting from reduced electric fields. Power-dependent photoluminescence spectroscopy of arrays of these nanorods confirms that their fabrication results in partial strain relaxation in the MQWs. Our technique allows us to interrogate the structures on a sufficiently small length scale to be able to extract the important information.},
keywords = {probing light emission, quantum wells, single nanorod, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/44537/}
}

High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 {\ensuremath{\mu}}m thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template.

Measurements of light emission from GaN nanorods of diameter between 80 and 350 nm, containing either a three-well multiple InGaN quantum well or a single quantum well, have been performed by photoluminescence (PL) and cathodoluminescence (CL) hyperspectral imaging. The PL underwent a Stark shift to the blue as the nanorod diameter was reduced, indicating substantial relaxation of the compressive strain in the quantum wells. The intensity of the nanorod emission per unit area can exceed that of the planar starting material. The CL measurements revealed that the wavelength of the quantum well emission varied with radial position in the nanorod. Simulations by a modal expansion method revealed that the light extraction efficiency varies with radial position and the variation is dependent on nanorod diameter. Finite difference time domain simulations showed that Bloch mode formation in the buffer layer below the nanorods impacts on the light extraction.

@Article{strathprints43862,
author = {Yi D. Zhuang and Szymon Lis and Jochen Bruckbauer and Simon E. J. O'Kane and Philip A. Shields and Paul R. Edwards and Jayanta Sarma and Robert W. Martin and Duncan W. E. Allsopp},
title = {Optical properties of GaN nanorods containing a single or multiple InGaN quantum wells},
journal = {Japanese Journal of Applied Physics},
year = {2013},
volume = {52},
pages = {08JE11},
month = {May},
abstract = {Measurements of light emission from GaN nanorods of diameter between 80 and 350 nm, containing either a three-well multiple InGaN quantum well or a single quantum well, have been performed by photoluminescence (PL) and cathodoluminescence (CL) hyperspectral imaging. The PL underwent a Stark shift to the blue as the nanorod diameter was reduced, indicating substantial relaxation of the compressive strain in the quantum wells. The intensity of the nanorod emission per unit area can exceed that of the planar starting material. The CL measurements revealed that the wavelength of the quantum well emission varied with radial position in the nanorod. Simulations by a modal expansion method revealed that the light extraction efficiency varies with radial position and the variation is dependent on nanorod diameter. Finite difference time domain simulations showed that Bloch mode formation in the buffer layer below the nanorods impacts on the light extraction.},
keywords = {optical properties, GaN nanorods, InGaN quantum wells, photoluminescence , cathodoluminescence, Physics, Atomic and Molecular Physics, and Optics},
url = {http://strathprints.strath.ac.uk/43862/}
}

Indium gallium nitride (In(x)Ga(1-x)N) is a technologically important material for many optoelectronic devices, including LEDs and solar cells, but it remains a challenge to incorporate high levels of InN into the alloy while maintaining sample quality. A series of InGaN epilayers was grown with different hydrogen flow rates (0-200 sccm) and growth temperatures (680-750 ?C) to obtain various InN fractions and bright emission in the range 390-480 nm. These 160-nm thick epilayers were characterized through several compositional techniques (wavelength dispersive x-ray spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry) and cathodoluminescence hyperspectral imaging. The compositional analysis with the different techniques shows good agreement when taking into account compositional gradients evidenced in these layers. The addition of small amounts of hydrogen to the gas flow at lower growth temperatures is shown to maintain a high surface quality and luminescence homogeneity. This allowed InN fractions of up to {\texttt{\char126}}16\% to be incorporated with minimal peak energy variations over a mapped area while keeping a high material quality.

@Article{strathprints43861,
author = {E Taylor and F Fang and F Oehler and P R Edwards and M J Kappers and K Lorenz and E Alves and C McAleese and C J Humphreys and R W Martin},
title = {Composition and luminescence studies of InGaN epilayers grown at different hydrogen flow rates},
journal = {Semiconductor Science and Technology},
year = {2013},
volume = {28},
number = {6},
pages = {065011},
month = {May},
abstract = {Indium gallium nitride (In(x)Ga(1-x)N) is a technologically important material for many optoelectronic devices, including LEDs and solar cells, but it remains a challenge to incorporate high levels of InN into the alloy while maintaining sample quality. A series of InGaN epilayers was grown with different hydrogen flow rates (0-200 sccm) and growth temperatures (680-750 ?C) to obtain various InN fractions and bright emission in the range 390-480 nm. These 160-nm thick epilayers were characterized through several compositional techniques (wavelength dispersive x-ray spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry) and cathodoluminescence hyperspectral imaging. The compositional analysis with the different techniques shows good agreement when taking into account compositional gradients evidenced in these layers. The addition of small amounts of hydrogen to the gas flow at lower growth temperatures is shown to maintain a high surface quality and luminescence homogeneity. This allowed InN fractions of up to {\texttt{\char126}}16\% to be incorporated with minimal peak energy variations over a mapped area while keeping a high material quality.},
keywords = {composition and luminescence studies, InGaN epilayers, hydrogen flow rates, optoelectronic devices, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/43861/}
}

A family of linear oligofluorene-BODIPY structures, containing either a ter- or quaterfluorene unit, have been prepared, in which the attachment of the oligofluorene chain to the BODIPY unit is switched between the meso-and beta-positions. Each member of this family was investigated by UV-vis absorption and photoluminescence spectroscopy, cyclic voltammetry and thermal studies (TGA and DSC) to determine their suitability as emissive layers in hybrid luminescent devices. One candidate was then successfully deployed as a down converter to convert UV to visible light.

@article{strathprints43346,
volume = {1},
number = {11},
title = {Linear oligofluorene-BODIPY structures for fluorescence applications},
author = {Neil J. Findlay and Clara Orofino-Pena and Jochen Bruckbauer and Saadeldin E. T. Elmasly and Sasikumar Arumugam and Anto R. Inigo and Alexander L. Kanibolotsky and Robert W. Martin and Peter J. Skabara},
year = {2013},
pages = {2249--2256},
journal = {Journal of Materials Chemistry. C},
keywords = {oligofluorene-BODIPY structures, cyclic voltammetry, thermal studies, Chemistry},
url = {http://strathprints.strath.ac.uk/43346/},
abstract = {A family of linear oligofluorene-BODIPY structures, containing either a ter- or quaterfluorene unit, have been prepared, in which the attachment of the oligofluorene chain to the BODIPY unit is switched between the meso-and beta-positions. Each member of this family was investigated by UV-vis absorption and photoluminescence spectroscopy, cyclic voltammetry and thermal studies (TGA and DSC) to determine their suitability as emissive layers in hybrid luminescent devices. One candidate was then successfully deployed as a down converter to convert UV to visible light.}
}

X-ray Reciprocal Space Mapping (RSM) is a powerful tool to explore the structure of semiconductor materials. However, conventional lab-based RSMs are usually measured in two dimensions (2D) ignoring the third dimension of diffraction-space volume. We report the use of a combination of X-ray microfocusing and state-of-the-art 2D area detectors to study the full volume of diffraction?space while probing III-nitride materials on the microscale.

2012

This talk reviews work on the optical properties of Eu-doped GaN at the Semiconductor Spectroscopy laboratory of the University of Strathclyde. The principal experimental technique used has been lamp-based Photoluminescence/ Excitation (PL/E) spectroscopy on samples produced mainly by high-energy ion implantation and annealing, either at low or high pressures of nitrogen, as described by Lorenz et al. [1]. These have been supplemented by samples doped in-situ either by Molecular Beam Epitaxy or Metallorganic Vapour Phase Epitaxy. Magneto-optic experiments on GaN:Eu were carried out in collaboration with the University of Bath.

@article{strathprints47951,
volume = {1342},
month = {January},
title = {Photoluminescence of Eu-doped GaN},
author = {Kevin Peter K.P. O'Donnell},
year = {2012},
pages = {101--109},
journal = {MRS Online Proceedings Library},
keywords = {semiconductor spectroscopy, Eu-doped GaN, high energy, vapour phase epitaxy, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/47951/},
abstract = {This talk reviews work on the optical properties of Eu-doped GaN at the Semiconductor Spectroscopy laboratory of the University of Strathclyde. The principal experimental technique used has been lamp-based Photoluminescence/ Excitation (PL/E) spectroscopy on samples produced mainly by high-energy ion implantation and annealing, either at low or high pressures of nitrogen, as described by Lorenz et al. [1]. These have been supplemented by samples doped in-situ either by Molecular Beam Epitaxy or Metallorganic Vapour Phase Epitaxy. Magneto-optic experiments on GaN:Eu were carried out in collaboration with the University of Bath.}
}

We are now all familiar with the bright blue, green and white LEDs that light up our electronic appliances; decorate our streets and buildings and illuminate airport runways. However, the ultimate performance of these nitride semiconductor based LEDs is limited by extended defects such as threading dislocations (TDs), partial dislocations (PDs) and stacking faults (SFs). If we want to develop LEDs to be an effective replacement for the light bulb, or have sufficient power to purify water; we need to eliminate these defects as they act as scattering centres for light and charge carriers and give rise to nonradiative recombination and to leakage currents, severely limiting device performance. The capability to rapidly detect and analyze TDs, PDs and SFs, with negligible sample preparation, represents a real step forward in the development of more efficient nitride-based semiconductor devices

@article{strathprints44510,
volume = {18},
number = {S2},
author = {Carol Trager-Cowan and Naresh Gunasekar and Benjamin Hourahine and Paul Edwards and Jochen Bruckbauer and Robert Martin and Christof Mauder and Austin Day and Gordon England and Aimo Winkelmann and Peter Parbrook and Anjus Wilkinson},
note = {Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 ? August 2, 2012.},
title = {Applications of electron channeling contrast imaging for characterizing nitride semiconductor thin films},
journal = {Microscopy and Microanalysis},
pages = {684--685},
year = {2012},
keywords = {semiconductor thin films , electron channeling, nanotechnology, Microbiology, Physics, Biotechnology},
url = {http://strathprints.strath.ac.uk/44510/},
abstract = {We are now all familiar with the bright blue, green and white LEDs that light up our electronic appliances; decorate our streets and buildings and illuminate airport runways. However, the ultimate performance of these nitride semiconductor based LEDs is limited by extended defects such as threading dislocations (TDs), partial dislocations (PDs) and stacking faults (SFs). If we want to develop LEDs to be an effective replacement for the light bulb, or have sufficient power to purify water; we need to eliminate these defects as they act as scattering centres for light and charge carriers and give rise to nonradiative recombination and to leakage currents, severely limiting device performance. The capability to rapidly detect and analyze TDs, PDs and SFs, with negligible sample preparation, represents a real step forward in the development of more efficient nitride-based semiconductor devices}
}

The design strategy presently employed to obtain ?white? light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ?true? red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1) the green gap in the internal quantum efficiency (IQE) of LEDs; (2) the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wavefunction overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.

@article{strathprints43970,
volume = {6},
number = {2},
month = {February},
author = {Kevin O'Donnell and Matthias Auf Der Maur and Aldo Di Carlo and Katharina Lorenz},
title = {It's not easy being green : strategies for all-nitrides, all-colour solid state lighting},
journal = {Physica Status Solidi (RRL) - Rapid Research Letters},
pages = {49--52},
year = {2012},
keywords = {solid state lighting, band engineering, nitrides, Solid state physics. Nanoscience, Materials Science(all), Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/43970/},
abstract = {The design strategy presently employed to obtain ?white? light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ?true? red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1) the green gap in the internal quantum efficiency (IQE) of LEDs; (2) the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wavefunction overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.}
}

We report on a new fabrication approach to create individually-addressable InGaN-based nanoscale-LEDs. It is based on the creation by LEEBI of a spatially confined sub-micron-size charge injection path within the p-GaN of an LED structure.

Magnetic semiconductors with coupled magnetic and electronic properties are of high technological and fundamental importance. Rare-earth elements can be used to introduce magnetic moments associated with the uncompensated spin of 4f-electrons into the semiconductor hosts. The luminescence produced by rare-earth doped semiconductors also attracts considerable interest due to the possibility of electrical excitation of characteristic sharp emission lines from intra 4f-shell transitions. Recently, electroluminescence of Eu-doped GaN in current-injection mode was demonstrated in p-n junction diode structures grown by organometallic vapour phase epitaxy. Unlike most other trivalent rare-earth ions, Eu3+ ions possess no magnetic moment in the ground state. Here we report the detection of an induced magnetic moment of Eu3+ ions in GaN which is associated with the 7F2 final state of 5D0{$\rightarrow$}7F2 optical transitions emitting at 622 nm. The prospect of controlling magnetic moments electrically or optically will lead to the development of novel magneto-optic devices.

@Article{strathprints42840,
author = {Vyacheslav V. Kachkanov and M. J. M.J. Wallace and Gerrit G. Van Der Laan and Sarnjeet S. S.S. Dhesi and Stuart A. S.A. Cavill and Yasufumi Y. Fujiwara and Kevin Peter K.P. O'Donnell},
title = {Induced magnetic moment of Eu³⁺ ions in GaN},
journal = {Scientific Reports},
year = {2012},
volume = {2},
pages = {969},
month = {December},
abstract = {Magnetic semiconductors with coupled magnetic and electronic properties are of high technological and fundamental importance. Rare-earth elements can be used to introduce magnetic moments associated with the uncompensated spin of 4f-electrons into the semiconductor hosts. The luminescence produced by rare-earth doped semiconductors also attracts considerable interest due to the possibility of electrical excitation of characteristic sharp emission lines from intra 4f-shell transitions. Recently, electroluminescence of Eu-doped GaN in current-injection mode was demonstrated in p-n junction diode structures grown by organometallic vapour phase epitaxy. Unlike most other trivalent rare-earth ions, Eu3+ ions possess no magnetic moment in the ground state. Here we report the detection of an induced magnetic moment of Eu3+ ions in GaN which is associated with the 7F2 final state of 5D0{$\rightarrow$}7F2 optical transitions emitting at 622 nm. The prospect of controlling magnetic moments electrically or optically will lead to the development of novel magneto-optic devices.},
keywords = {materials science, optical materials, applied physics, Physics, General},
url = {http://strathprints.strath.ac.uk/42840/}
}

GaN epilayers were implanted with Eu to fluences of 1{$\times$}1013 Eu/cm2 and 1{$\times$}1015 Eu/cm2. Post-implant thermal annealing was performed in ultra-high nitrogen pressures at temperatures up to 1450 ?C. For the lower fluence effective structural recovery of the crystal was observed for annealing at 1000 ?C while optical activation could be further improved at higher annealing temperatures. The higher fluence samples also reveal good optical activation; however, some residual implantation damage remains even for annealing at 1450 ?C which leads to a reduced incorporation of Eu on substitutional sites, a broadening of the Eu luminescence lines and to a strongly reduced fraction of optically active Eu ions. Possibilities for further optimization of implantation and annealing conditions are discussed.

Anisotropy of the valence band is experimentally demonstrated in CuInSe2, a key component of the absorber layer in one of the leading thin-film solar cell technology. By changing the orientation of applied magnetic fields with respect to the crystal lattice, we measure considerable differences in the diamagnetic shifts and effective g-factors for the A and B free excitons. The resulting free exciton reduced masses are combined with a perturbation model for non-degenerate independent excitons and theoretical dielectric constants to provide the anisotropic effective hole masses, revealing anisotropies of 5.5 (4.2) for the A (B) valence bands.

Evolution of the valence-band structure at gradually increasing copper content has been analysed by x-ray photoelectron spectroscopy (XPS) in In2Se3, CuIn5Se8, CuIn3Se5, and CuInSe2 single crystals. A comparison of these spectra with calculated total and angular-momentum resolved density-of-states (DOS) revealed the main trends of this evolution. The formation of the theoretically predicted gap between the bonding and non-bonding states has been observed in both experimental XPS spectra and theoretical DOS.

Hyperspectral cathodoluminescence imaging provides spectrally and spatially resolved information on luminescent materials within a single dataset. Pushing the technique toward its ultimate nanoscale spatial limit, while at the same time spectrally dispersing the collected light before detection, increases the challenge of generating low-noise images. This article describes aspects of the instrumentation, and in particular data treatment methods, which address this problem. The methods are demonstrated by applying them to the analysis of nanoscale defect features and fabricated nanostructures in III-nitride-based materials.

The design strategy presently employed to obtain ‘white’ light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ‘true’ red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wave-function overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.

@article{strathprints42405,
volume = {6},
number = {2},
month = {February},
author = {K. P. O'Donnell and M. Auf Der Maur and A. Di Carlo and K. Lorenz and SORBET Consortium },
title = {It's not easy being green : strategies for all-nitrides, all-colour solid state lighting},
journal = {Physica Status Solidi (RRL) - Rapid Research Letters},
pages = {49--52},
year = {2012},
keywords = {III-nitrides, light emitting diodes, bandgap engineering, solid state lighting, Physics, Materials Science(all), Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/42405/},
abstract = {The design strategy presently employed to obtain 'white' light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining 'true' red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wave-function overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.}
}

The properties of InGaN alloys are important for many applications in optoelectronics, since their fundamental gap spans the visible range. Calculating properties, particularly for InN, is theoretically challenging, especially to obtain accurate values for the band gap. We have developed a semi-empirical parametrization of (In,Ga) N using the density functional based tight binding method (DFTB), where the band gaps of InN and GaN have been empirically corrected to experiment. We demonstrate the performance of this method by calculating a range of properties for the two materials, including elastic constants and carrier effective masses. There are several methods to model alloys of these material, one of the simplest being the virtual crystal approximation, which we apply to derive electronic properties over the whole composition range for InGaN.

@Article{strathprints42202,
author = {Fathi Elfituri and Benjamin Hourahine},
title = {Simple models for InGaN alloys},
journal = {Physica Status Solidi A},
year = {2012},
volume = {209},
number = {1},
pages = {79--82},
abstract = {The properties of InGaN alloys are important for many applications in optoelectronics, since their fundamental gap spans the visible range. Calculating properties, particularly for InN, is theoretically challenging, especially to obtain accurate values for the band gap. We have developed a semi-empirical parametrization of (In,Ga) N using the density functional based tight binding method (DFTB), where the band gaps of InN and GaN have been empirically corrected to experiment. We demonstrate the performance of this method by calculating a range of properties for the two materials, including elastic constants and carrier effective masses. There are several methods to model alloys of these material, one of the simplest being the virtual crystal approximation, which we apply to derive electronic properties over the whole composition range for InGaN.},
keywords = {InGaN alloys, optoelectronics , crystal approximation, Physics, Materials Chemistry, Surfaces, Coatings and Films, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/42202/}
}

The promise of the broad range of direct band gaps of the \{Al,Ga,In\}N system is limited by the crystal quality of current material. As grown defect densities of InN, when compared with the more mature GaN, are extremely high and InN is strongly influenced by these defects. This is particularly important due to the unusual position of the charge neutrality level of InN, leading to both the well known surface charge accumulation and difficulties in p-type doping. While impurities and native defects clearly impact on the bulk carrier density in InN, the effects of threading dislocations on the electrical properties are still in dispute. Issues such as whether the dislocation line is charged or contains dangling bonds remain open. We present the results of a global search for possible dislocation core reconstructions for a range of screw dislocations in wurtzite III-N material, utilizing empirical Stillinger-Weber inter-atomic potentials. In addition we investigate a wide range of non-stoichiometric core structures.

@article{strathprints42201,
volume = {209},
number = {1},
month = {January},
author = {Simon Kraeusel and Benjamin Hourahine},
title = {Global search for stable screw dislocation cores in III-N semiconductors},
journal = {Physica Status Solidi A},
pages = {71--74},
year = {2012},
keywords = {semiconductors , stable screw dislocation , crystal structures, Solid state physics. Nanoscience, Materials Chemistry, Surfaces, Coatings and Films, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/42201/},
abstract = {The promise of the broad range of direct band gaps of the \{Al,Ga,In\}N system is limited by the crystal quality of current material. As grown defect densities of InN, when compared with the more mature GaN, are extremely high and InN is strongly influenced by these defects. This is particularly important due to the unusual position of the charge neutrality level of InN, leading to both the well known surface charge accumulation and difficulties in p-type doping. While impurities and native defects clearly impact on the bulk carrier density in InN, the effects of threading dislocations on the electrical properties are still in dispute. Issues such as whether the dislocation line is charged or contains dangling bonds remain open. We present the results of a global search for possible dislocation core reconstructions for a range of screw dislocations in wurtzite III-N material, utilizing empirical Stillinger-Weber inter-atomic potentials. In addition we investigate a wide range of non-stoichiometric core structures.}
}

We study the electromagnetic response of smooth gold nanoparticles with shapes varying from a single sphere to two ellipsoids joined smoothly at their vertices. We show that the plasmonic resonance visible in the extinction and absorption cross sections shifts to longer wavelengths and eventually disappears as the mid-plane waist of the composite particle becomes narrower. This process corresponds to an increase of the numbers of internal and scattering modes that are mainly confined to the surface and coupled to the incident field. These modes strongly affect the near field, and therefore are of great importance in surface spectroscopy, but are almost undetectable in the far field.

@Article{strathprints42181,
author = {B. Hourahine and F. Papoff},
title = {The geometrical nature of optical resonances : from a sphere to fused dimer nanoparticles},
journal = {Measurement Science and Technology},
year = {2012},
volume = {23},
number = {8},
pages = {084002},
month = {August},
abstract = {We study the electromagnetic response of smooth gold nanoparticles with shapes varying from a single sphere to two ellipsoids joined smoothly at their vertices. We show that the plasmonic resonance visible in the extinction and absorption cross sections shifts to longer wavelengths and eventually disappears as the mid-plane waist of the composite particle becomes narrower. This process corresponds to an increase of the numbers of internal and scattering modes that are mainly confined to the surface and coupled to the incident field. These modes strongly affect the near field, and therefore are of great importance in surface spectroscopy, but are almost undetectable in the far field.},
keywords = {geometrical nature , optical resonances , sphere , fused dimer , Physics, Instrumentation, Applied Mathematics},
url = {http://strathprints.strath.ac.uk/42181/}
}

We report a new approach for accurate calculation of optical cross sections and internal and scattered fields at any point in space for micro- and nanoparticles. Our approach is based on constructing the intrinsic optical modes of general smooth particles and hence optimised surface Green functions that, for any incident field, provide an a priori upper bound on the error and identify the class of incident fields with largest error.

Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709448]

@Article{strathprints40877,
author = {F. Luckert and M. V. Yakushev and C. Faugeras and A. V. Karotki and A. V. Mudryi and R. W. Martin},
title = {Excitation power and temperature dependence of excitons in CuInSe₂},
journal = {Journal of Applied Physics},
year = {2012},
volume = {111},
number = {9},
pages = {093507},
month = {May},
abstract = {Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709448]},
keywords = {band-edge photoluminescence, acceptors, CulnSe2, luminescence, emission, defect physics, radiative recombination, chalcopyrite semiconductor, single-crystals, excitation power , temperature dependence , excitons, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/40877/}
}

There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1-xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (similar to 10 mu m) zinc-blende and wurtzite AlxGa1-xN films were grown by PA-MBE on 2-in. GaAs (0 0 1) and GaAs (1 1 1)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1-xN wafers can be achieved by PA-MBE for a wide range of Al compositions. (C) 2011 Elsevier B.V. All rights reserved.

Our study of samples grown in different metalorganic chemical vapor deposition reactors and with different growth conditions reveals that V-pits are always present in (InxAl1-x)N films whatever the layer thickness and the InN content. V-pits are empty inverted pyramids terminating threading dislocations. InN-rich triangular regions are present around the threading dislocations terminated by pits with a hexagonal 6-fold symmetry distribution in \{11 – 20\} planes. The nature of the facets of the V-pits depends on the growth conditions: pits with either \{11 – 2l\}, I being between 1 and 3, or \{1 – 101\} facets have been observed. Moreover, the nature of the threading dislocations terminated by pits also depends on the growth conditions. Our observations suggest that with a high V/III ratio only edge a + c-type dislocations are terminated by pits whereas with a low V/III ratio both edge a-type and mixed a + c-type dislocations are terminated by pits.

The optical properties of GaN layers coalesced above an array of nanocolumns have important consequences for advanced optoelectronic devices. GaN nanocolumns coalesced using a nanoscale epitaxial overgrowth technique have been investigated by high resolution cathodoluminescence (CL) hyperspectral imaging. Plan-view microscopy reveals partially coalesced GaN layers with a sub-{\ensuremath{\mu}}m scale domain structure and distinct grain boundaries, which is mapped using CL spectroscopy showing high strain at the grain boundaries. Cross-sectional areas spanning the partially coalesced GaN and underlying nanocolumns are mapped using CL, revealing that the GaN bandedge peak shifts by about 25 meV across the partially coalesced layer of {$\sim$}2 {\ensuremath{\mu}}m thick. The GaN above the nanocolumns remains under tensile strain, probably due to Si out-diffusion from the mask or substrate. The cross-sectional data show how this strain is reduced towards the surface of the partially coalesced layer, possibly due to misalignment between adjacent partially coalesced regions. {\copyright} 2012 American Institute of Physics

A size-dependent strain relaxation and its effects on the optical properties of InGaN/GaN multiple quantum wells (QWs) in micro-pillars have been investigated through a combination of high spatial resolution cathodoluminescence (CL) hyperspectral imaging and numerical modeling. The pillars have diameters (d) ranging from 2 to 150 {\ensuremath{\mu}}m and were fabricated from a III-nitride light-emitting diode (LED) structure optimized for yellow-green emission at {$\sim$}560 nm. The CL mapping enables us to investigate strain relaxation in these pillars on a sub-micron scale and to confirm for the first time that a narrow ({$\leq$}2 {\ensuremath{\mu}}m) edge blue-shift occurs even for the large InGaN/GaN pillars (d {\ensuremath{>}} 10 {\ensuremath{\mu}}m). The observed maximum blue-shift at the pillar edge exceeds 7 nm with respect to the pillar centre for the pillars with diameters in the 2?16 {\ensuremath{\mu}}m range. For the smallest pillar (d = 2 {\ensuremath{\mu}}m), the total blue-shift at the edge is 17.5 nm including an 8.2 nm ?global? blue-shift at the pillar centre in comparison with the unetched wafer. By using a finite element method with a boundary condition taking account of a strained GaN buffer layer which was neglected in previous simulation works, the strain distribution in the QWs of these pillars was simulated as a function of pillar diameter. The blue-shift in the QWs emission wavelength was then calculated from the strain-dependent changes in piezoelectric field, and the consequent modification of transition energy in the QWs. The simulation and experimental results agree well, confirming the necessity for considering the strained buffer layer in the strain simulation. These results provide not only significant insights into the mechanism of strain relaxation in these micro-pillars but also practical guidance for design of micro/nano LEDs.

@Article{strathprints40400,
author = {E. Y. Xie and Z. Z. Chen and P. R. Edwards and Z. Gong and N. Y. Liu and Y. B. Tao and Y. F. Zhang and Y. J. Chen and I. M. Watson and E. Gu and R. W. Martin and G. Y. Zhang and M. D. Dawson},
title = {Strain relaxation in InGaN/GaN micro-pillars evidenced by high resolution cathodoluminescence hyperspectral imaging},
journal = {Journal of Applied Physics},
year = {2012},
volume = {112},
number = {1},
pages = {013107},
month = {July},
abstract = {A size-dependent strain relaxation and its effects on the optical properties of InGaN/GaN multiple quantum wells (QWs) in micro-pillars have been investigated through a combination of high spatial resolution cathodoluminescence (CL) hyperspectral imaging and numerical modeling. The pillars have diameters (d) ranging from 2 to 150 {\ensuremath{\mu}}m and were fabricated from a III-nitride light-emitting diode (LED) structure optimized for yellow-green emission at {$\sim$}560 nm. The CL mapping enables us to investigate strain relaxation in these pillars on a sub-micron scale and to confirm for the first time that a narrow ({$\leq$}2 {\ensuremath{\mu}}m) edge blue-shift occurs even for the large InGaN/GaN pillars (d {\ensuremath{>}} 10 {\ensuremath{\mu}}m). The observed maximum blue-shift at the pillar edge exceeds 7 nm with respect to the pillar centre for the pillars with diameters in the 2?16 {\ensuremath{\mu}}m range. For the smallest pillar (d = 2 {\ensuremath{\mu}}m), the total blue-shift at the edge is 17.5 nm including an 8.2 nm ?global? blue-shift at the pillar centre in comparison with the unetched wafer. By using a finite element method with a boundary condition taking account of a strained GaN buffer layer which was neglected in previous simulation works, the strain distribution in the QWs of these pillars was simulated as a function of pillar diameter. The blue-shift in the QWs emission wavelength was then calculated from the strain-dependent changes in piezoelectric field, and the consequent modification of transition energy in the QWs. The simulation and experimental results agree well, confirming the necessity for considering the strained buffer layer in the strain simulation. These results provide not only significant insights into the mechanism of strain relaxation in these micro-pillars but also practical guidance for design of micro/nano LEDs.},
keywords = {cathodoluminescence, light emitting diodes, spectral line shift, quantum well devices, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/40400/}
}

Coalescence of GaN over arrays of GaN nanopyramids has important device applications and has been achieved on nano-imprint lithographically patterned GaN/sapphire substrates using metal organic vapour phase epitaxy. Spatially and spectrally resolved cathdoluminescence (CL) from such coalesced layers are studied in detail. The observed redshift of the GaN band edge emission with increasing electron beam depth of maximum CL into the coalesced layer is discussed in relation to a carrier-induced peak shift, likely due to Si out-diffusion from the mask material into the GaN. Depth-resolved CL measurements are used to quantify the redshift in terms of bandgap renormalization and strain effects. CL maps showing the GaN near band edge peak energy distribution reveal micron-scale domain-like variations in peak energy and are attributed to the effects of local strain.

@Article{strathprints40398,
author = {K J Lethy and P R Edwards and C Liu and P A Shields and D W E Allsopp and R W Martin},
title = {Cathodoluminescence studies of GaN coalesced from nanopyramids selectively grown by MOVPE},
journal = {Semiconductor Science and Technology},
year = {2012},
volume = {27},
number = {8},
pages = {085010},
month = {June},
abstract = {Coalescence of GaN over arrays of GaN nanopyramids has important device applications and has been achieved on nano-imprint lithographically patterned GaN/sapphire substrates using metal organic vapour phase epitaxy. Spatially and spectrally resolved cathdoluminescence (CL) from such coalesced layers are studied in detail. The observed redshift of the GaN band edge emission with increasing electron beam depth of maximum CL into the coalesced layer is discussed in relation to a carrier-induced peak shift, likely due to Si out-diffusion from the mask material into the GaN. Depth-resolved CL measurements are used to quantify the redshift in terms of bandgap renormalization and strain effects. CL maps showing the GaN near band edge peak energy distribution reveal micron-scale domain-like variations in peak energy and are attributed to the effects of local strain.},
keywords = {cathodoluminescence, ionoluminescence , nanoparticles, nanolithography, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/40398/}
}

We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of ?seeds? on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth.

@Article{strathprints40397,
author = {V. Kachkanov and I. P. Dolbnya and K. P. O'Donnell and K. Lorenz and S. Pereira and R. W. Martin and P. R. Edwards and I. M. Watson},
title = {Characterization of InGaN and InAlN epilayers by microdiffraction X-Ray reciprocal space mapping},
journal = {MRS Online Proceedings Library},
year = {2012},
volume = {1396},
pages = {mrsf11-1396-o06-11},
month = {February},
abstract = {We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of ?seeds? on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth.},
keywords = {microstructure, nitride, x-ray diffraction (XRD), Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/40397/}
}

The potential colour rendering capability and efficiency of white LEDs constructed by a combination of individual red, green and blue (RGB) LEDs are analysed. The conventional measurement of colour rendering quality, the colour rendering index (CRI), is used as well as a recently proposed colour quality scale (CQS), designed to overcome some of the limitations of CRI when narrow-band emitters are being studied. The colour rendering performance is maximised by variation of the peak emission wavelength and relative intensity of the component LEDs, with the constraint that the spectral widths follow those measured in actual devices. The highest CRI achieved is 89.5, corresponding to a CQS value of 79, colour temperature of 3800 K and a luminous efficacy of radiation (LER) of 365 lm/W. By allowing the spectral width of the green LED to vary the CRI can be raised to 90.9, giving values of 82.5 and 370 lm/W for the CQS and LER, respectively. The significance of these values are discussed in terms of optimising the possible performance of RGB LEDs.

@article{strathprints39520,
volume = {209},
number = {3},
month = {February},
author = {Elaine Taylor and Paul Edwards and Robert Martin},
title = {Colorimetry and efficiency of white LEDs : Spectral width dependence},
journal = {Physica Status Solidi A: Applications and Materials Science},
pages = {461--464},
year = {2012},
keywords = {spectral width, colorimetry, GaN, LED, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/39520/},
abstract = {The potential colour rendering capability and efficiency of white LEDs constructed by a combination of individual red, green and blue (RGB) LEDs are analysed. The conventional measurement of colour rendering quality, the colour rendering index (CRI), is used as well as a recently proposed colour quality scale (CQS), designed to overcome some of the limitations of CRI when narrow-band emitters are being studied. The colour rendering performance is maximised by variation of the peak emission wavelength and relative intensity of the component LEDs, with the constraint that the spectral widths follow those measured in actual devices. The highest CRI achieved is 89.5, corresponding to a CQS value of 79, colour temperature of 3800 K and a luminous efficacy of radiation (LER) of 365 lm/W. By allowing the spectral width of the green LED to vary the CRI can be raised to 90.9, giving values of 82.5 and 370 lm/W for the CQS and LER, respectively. The significance of these values are discussed in terms of optimising the possible performance of RGB LEDs.}
}

We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than {$\sim$}100 ?C under both Ga-rich and N-rich growth conditions. The formation of amorphous GaN1?xBix alloys is promoted by growth under Ga-rich conditions. The amorphous matrix has a short-range order resembling random crystalline GaN1?xBix alloys. We have observed the formation of small crystalline clusters embedded into amorphous GaN1?xBix alloys. Despite the fact that the films are pseudo-amorphous we observe a well defined optical absorption edges that rapidly shift to very low energy of {$\sim$}1 eV.

We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120? to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.

@Article{strathprints39229,
author = {Naresh Gunasekar and Benjamin Hourahine and Paul Edwards and A.P. Day and Aimo Winkelmann and A.J. Wilkinson and P.J. Parbrook and G. England and Carol Trager-Cowan},
title = {Rapid nondestructive analysis of threading dislocations in wurtzite materials using the scanning electron microscope},
journal = {Physical Review Letters},
year = {2012},
volume = {108},
number = {13},
pages = {135503},
month = {March},
abstract = {We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120? to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.},
keywords = {wurtzite , nondestructive analysis , scanning electron microscope, Solid state physics. Nanoscience, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/39229/}
}

The nature and interplay of intrinsic point and extended defects in n-type Si-doped InN epilayers with free carrier concentrations up to 6.6{$\times$}1020 cm?3 are studied using positron annihilation spectroscopy and transmission electron microscopy and compared to results from undoped irradiated films. In as-grown Si-doped samples, mixed In-N vacancy complexes (VIn-VN) are the dominant III-sublattice related vacancy defects. An increase in the number of VN in these complexes toward the interface suggests high concentrations of additional isolated VN and VN-clusters near the GaN buffer layer and coincides with elevated dislocation densities in that area.

We describe the use of electron channelling contrast imaging (ECCI) ? in a field emission scanning electron microscope ? to reveal and identify defects in nitride semiconductor thin films. In ECCI changes in crystallographic orientation, or changes in lattice constant due to local strain, are revealed by changes in grey scale in an image constructed by monitoring the intensity of backscattered electrons as an electron beam is scanned over a suitably oriented sample. Extremely small orientation changes are detectable, enabling small angle tilt and rotation boundaries and dislocations to be imaged. Images with a resolution of tens of nanometres are obtainable with ECCI. In this paper we describe the use of ECCI with TEM to determine threading dislocation densities and types in InAlN/GaN heterostructures grown on SiC and sapphire substrates.

@article{strathprints35178,
volume = {209},
number = {3},
month = {March},
author = {G. Naresh-Kumar and Benjamin Hourahine and A. Vilalta-Clemente and P. Ruterana and P. Gamarra and C. Lacam and M. Tordjman and M. A. di Forte-Poisson and P. J. Parbrook and A. P. Day and G. England and Carol Trager-Cowan},
title = {Imaging and identifying defects in nitride semiconductor thin films using a scanning electron microscope},
journal = {Physica Status Solidi A},
pages = {424--426},
year = {2012},
keywords = {dislocations, electron channelling , SEM, Solid state physics. Nanoscience, Materials Chemistry, Surfaces, Coatings and Films, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/35178/},
abstract = {We describe the use of electron channelling contrast imaging (ECCI) ? in a field emission scanning electron microscope ? to reveal and identify defects in nitride semiconductor thin films. In ECCI changes in crystallographic orientation, or changes in lattice constant due to local strain, are revealed by changes in grey scale in an image constructed by monitoring the intensity of backscattered electrons as an electron beam is scanned over a suitably oriented sample. Extremely small orientation changes are detectable, enabling small angle tilt and rotation boundaries and dislocations to be imaged. Images with a resolution of tens of nanometres are obtainable with ECCI. In this paper we describe the use of ECCI with TEM to determine threading dislocation densities and types in InAlN/GaN heterostructures grown on SiC and sapphire substrates.}
}

The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 {\ensuremath{<}} x {\ensuremath{<}} 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95.

@Article{strathprints44735,
author = {R. E. L. Powell and S. V. Novikov and F. Luckert and P. R. Edwards and A. V. Akimov and C. T. Foxon and R. W. Martin and A. J. Kent},
title = {Carrier localization and related photoluminescence in cubic AlGaN epilayers},
journal = {Journal of Applied Physics},
year = {2011},
volume = {110},
number = {6},
pages = {063517},
month = {September},
abstract = {The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 {\ensuremath{<}} x {\ensuremath{<}} 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95.},
keywords = {solid-solutions, alloys, epilayers, spectra, Physics, Atomic and Molecular Physics, and Optics},
url = {http://strathprints.strath.ac.uk/44735/}
}

The optical properties of ternary chalcopyrite AgGaTe2 were studied by photoreflectance spectorscopy (PR). Due to the optimal direct energy gap and high absorption coefficient AgGaTe2 is a promising material for solar energy conversion. Single crystals used in this work were grown by the vertical Bridgman technique. The PR temperature dependent spectra were measured in the range of 25?300 K. At room temperature two energy gaps in AgGaTe2 were detected: and , with temperature coefficients /dT=?2.1{$\times$}10?4 eV/K and /dT=?3.4{$\times$}10?4 eV/K. At low temperature (T=25 K) these bandgap energies were and . Temperature dependence of bandgap energies is maximum at about T=90 K.

Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu(Ga), is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N(2). According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu(Ga) in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the (5)D(0) to (7)F(2) spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the ‘host-independent’ excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.

@article{strathprints35792,
volume = {33},
number = {7},
month = {May},
author = {K. P. O'Donnell and I. S. Roqan and Ke Wang and K. Lorenz and E. Alves and M. Bockowski},
title = {The photoluminescence/excitation (PL/E) spectroscopy of Eu-implanted GaN},
journal = {Optical Materials},
pages = {1063--1065},
year = {2011},
keywords = {ion implantation, excitation mechanism, europium, luminescence, photoluminescence, gallium nitride, photoluminescence/excitation , PL/E, spectroscopy, Eu-implanted GaN , Physics, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics, Electrical and Electronic Engineering, Computer Science(all)},
url = {http://strathprints.strath.ac.uk/35792/},
abstract = {Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu(Ga), is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N(2). According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu(Ga) in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the (5)D(0) to (7)F(2) spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the 'host-independent' excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.}
}

We report the magnetic field splittings of emission lines assigned to the 5D0?7F2 transitions of Eu3+ centres in GaN. The application of a magnetic field in the c-axis direction (B{\ensuremath{|}}{\ensuremath{|}}c) leads to a splitting of the major lines at 621 nm, 622 nm and 622.8 nm into two components. The Zeeman splitting is linear with magnetic field up to 5 Tesla for each line. In contrast, a magnetic field applied in the growth plane (B?c) does not influence the photoluminescence spectra. The estimated g-factors vary slightly from sample to sample with mean values of g{\ensuremath{|}}{\ensuremath{|}} {\texttt{\char126}}2.8, {\texttt{\char126}}1.5 and {\texttt{\char126}}2.0 for the emission lines at 621 nm, 622 nm and 622.8 nm respectively.

@Article{strathprints35791,
author = {V. Kachkanov and K. P. O'Donnell and C. Rice and D. Wolverson and R. W. Martin and K. Lorenz and E. Alves and M. Bockowski},
title = {Zeeman splittings of the {⁵D₀}--{⁷F₂} transitions of Eu³⁺ ions implanted into GaN},
journal = {MRS Online Proceedings Library},
year = {2011},
volume = {1290},
pages = {mrsf10-1290-i03-06},
abstract = {We report the magnetic field splittings of emission lines assigned to the 5D0?7F2 transitions of Eu3+ centres in GaN. The application of a magnetic field in the c-axis direction (B{\ensuremath{|}}{\ensuremath{|}}c) leads to a splitting of the major lines at 621 nm, 622 nm and 622.8 nm into two components. The Zeeman splitting is linear with magnetic field up to 5 Tesla for each line. In contrast, a magnetic field applied in the growth plane (B?c) does not influence the photoluminescence spectra. The estimated g-factors vary slightly from sample to sample with mean values of g{\ensuremath{|}}{\ensuremath{|}} {\texttt{\char126}}2.8, {\texttt{\char126}}1.5 and {\texttt{\char126}}2.0 for the emission lines at 621 nm, 622 nm and 622.8 nm respectively.},
keywords = {Eu, ion-implantation, nitride, TIC - Bionanotechnology, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/35791/}
}

We report the use of microfocused three-dimensional x-ray reciprocal space mapping to study InGaN epilayers with average InN content 20\%-22\%. Analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused x-ray beam, allowed us to gain valuable information about the nanostructure of InN-rich InGaN epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain-free. The initial stages of InGaN-on-GaN epitaxial growth, therefore, conform to the Volmer-Weber growth mechanism with ?seeds? nucleated on strain fields generated by the a-type edge dislocations.

We give a geometrical theory of resonances in Maxwell?s equations that generalizes the Mie formulae for spheres to all scattering channels of any dielectric or metallic particle without sharp edges. We show that the electromagnetic response of a particle is given by a set of modes of internal and scattered fields that are coupled pairwise on the surface of the particle and reveal that resonances in nanoparticles and excess noise in macroscopic cavities have the same origin. We give examples of two types of optical resonances: those in which a single pair of internal and scattered modes become strongly aligned in the sense defined in this paper, and those resulting from constructive interference of many pairs of weakly aligned modes, an effect relevant for sensing. This approach calculates resonances for every significant mode of particles, demonstrating that modes can be either bright or dark depending on the incident field. Using this extra mode information we then outline how excitation can be optimized. Finally, we apply this theory to gold particles with shapes often used in experiments, demonstrating effects including a Fano-like resonance.

@article{strathprints34708,
volume = {19},
number = {22},
month = {October},
author = {Francesco Papoff and Benjamin Hourahine},
title = {Geometrical Mie theory for resonances in nanoparticles of any shape},
journal = {Optics Express},
pages = {21432--21444},
year = {2011},
keywords = {metals, scattering, nanomaterials, scattering theory, Physics, Atomic and Molecular Physics, and Optics},
url = {http://strathprints.strath.ac.uk/34708/},
abstract = {We give a geometrical theory of resonances in Maxwell?s equations that generalizes the Mie formulae for spheres to all scattering channels of any dielectric or metallic particle without sharp edges. We show that the electromagnetic response of a particle is given by a set of modes of internal and scattered fields that are coupled pairwise on the surface of the particle and reveal that resonances in nanoparticles and excess noise in macroscopic cavities have the same origin. We give examples of two types of optical resonances: those in which a single pair of internal and scattered modes become strongly aligned in the sense defined in this paper, and those resulting from constructive interference of many pairs of weakly aligned modes, an effect relevant for sensing. This approach calculates resonances for every significant mode of particles, demonstrating that modes can be either bright or dark depending on the incident field. Using this extra mode information we then outline how excitation can be optimized. Finally, we apply this theory to gold particles with shapes often used in experiments, demonstrating effects including a Fano-like resonance.}
}

A method to calculate the multiplet states of lanthanide impurities in solids is presented. This approach is based on a semi-empirical density functional method which includes corrections to account for the correlation and spin-orbit coupling of the 4f electrons. Specific multiplet states of the rare earth are produced by constraining the system. This approach is then used to investigate some of the properties of substitutional europium impurities in gallium nitride, reproducing the relative energy of two multiplets, and discussing a potential excitation mechanism for these centers.

@Article{strathprints34305,
author = {Benjamin Hourahine},
title = {Excited multiplets of Eu in GaN},
journal = {MRS Online Proceedings Library},
year = {2011},
volume = {1290},
pages = {46--51},
abstract = {A method to calculate the multiplet states of lanthanide impurities in solids is presented. This approach is based on a semi-empirical density functional method which includes corrections to account for the correlation and spin-orbit coupling of the 4f electrons. Specific multiplet states of the rare earth are produced by constraining the system. This approach is then used to investigate some of the properties of substitutional europium impurities in gallium nitride, reproducing the relative energy of two multiplets, and discussing a potential excitation mechanism for these centers.},
keywords = {Eu, GaN, electronic structure, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/34305/}
}

CuInSe2 single crystals, grown by the vertical Bridgman technique were studied using polarisation resolved photoluminescence (PL) at cryogenic temperatures. The emission lines related to the first (n = 2) excited states for the A and B free excitons were observed in the PL spectra at 1.0481 and 1.0516 eV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (E-g(A) = 1.050 eV and E-g(B) = 1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.

@Article{strathprints34032,
author = {Michael V. Yakushev and Franziska Luckert and Clement Faugeras and Anatoli V. Karotki and Alexander V. Mudryi and Robert W. Martin},
title = {Excited states of the A and B free excitons in CuInSe₂},
journal = {Japanese Journal of Applied Physics},
year = {2011},
volume = {50},
number = {5},
pages = {05FC03},
month = {May},
abstract = {CuInSe2 single crystals, grown by the vertical Bridgman technique were studied using polarisation resolved photoluminescence (PL) at cryogenic temperatures. The emission lines related to the first (n = 2) excited states for the A and B free excitons were observed in the PL spectra at 1.0481 and 1.0516 eV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (E-g(A) = 1.050 eV and E-g(B) = 1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.},
keywords = {optical properties, single crystals, Physics},
url = {http://strathprints.strath.ac.uk/34032/}
}

Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.

Solar cells with the structure ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se-2/Mo/polyimide were examined using a range of techniques. The elemental composition of the Cu(InGa)Se-2 (CIGS) layers, their crystalline structure and optical properties were studied. Photoluminescence (PL) spectra of the CIGS absorber layers were studied as functions of temperature (4.2-240 K) and excitation power density. The band gap energy E-g of the CIGS layers was determined by employing photoluminescence excitation (PLE) spectroscopy. The influence of sodium incorporation on the PL properties of CIGS was analysed. Correlations of the optical properties of the CIGS absorber layers and the photovoltaic parameters of the solar cells were revealed.

Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 {\ensuremath{<}} x {\ensuremath{<}} 0.8) on both crystalline (sapphire and silicon) and amorphous (glass and Pyrex glass) substrates. Despite the fact that the samples with high As content are amorphous, we observe a gradual continuous decrease of bandgap from similar to 3.4 to similar to 0.8 eV with increase in As content. To the best of our knowledge this is the first demonstration of homogeneous amorphous GaN-based alloys over a wide composition range. The large band gap range of the amorphous phase of GaNAs covers much of the solar spectrum. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass and Pyrex glass can be used for solar cell fabrication.

@article{strathprints32272,
volume = {323},
number = {1},
month = {May},
author = {S.V. Novikov and C.R. Staddon and C.T. Foxon and K.M. Yu and R. Broesler and M. Hawkridge and Z. Liliental-Weber and J. Denlinger and I. Demchenko and Franziska Luckert and Paul Edwards and Robert Martin and W. Walukiewicz},
title = {Growth by molecular beam epitaxy of amorphous and crystalline GaNAs alloys with band gaps from 3.4 to 0.8 eV for solar energy conversion devices},
journal = {Journal of Crystal Growth},
pages = {60--63},
year = {2011},
keywords = {crystals, molecular beam epitaxy, amorphous, crystalline, GaNAs alloys , solar energy conversion devices, band gaps , Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/32272/},
abstract = {Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 {\ensuremath{<}} x {\ensuremath{<}} 0.8) on both crystalline (sapphire and silicon) and amorphous (glass and Pyrex glass) substrates. Despite the fact that the samples with high As content are amorphous, we observe a gradual continuous decrease of bandgap from similar to 3.4 to similar to 0.8 eV with increase in As content. To the best of our knowledge this is the first demonstration of homogeneous amorphous GaN-based alloys over a wide composition range. The large band gap range of the amorphous phase of GaNAs covers much of the solar spectrum. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass and Pyrex glass can be used for solar cell fabrication.}
}

We have studied the growth of zinc-blende GaN and AlxGa1-xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100 mu m in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (0 0 1) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1-xN wafers.

@article{strathprints32270,
volume = {323},
number = {1},
month = {May},
author = {S.V. Novikov and C.R. Staddon and C.T. Foxon and Franziska Luckert and Paul Edwards and Robert Martin and A.J. Kent},
title = {Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals},
journal = {Journal of Crystal Growth},
pages = {80--83},
year = {2011},
keywords = {substrates, molecular beam epitaxy, nitrides, semiconducting III?V materials, free-standing bulk , zinc-blende GaN , AlGaN crystals, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/32270/},
abstract = {We have studied the growth of zinc-blende GaN and AlxGa1-xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100 mu m in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (0 0 1) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1-xN wafers.}
}

Localized surface plasmons within silver nanocubes less than 50 nm in size are investigated using high resolution cathodoluminescence hyperspectral imaging. Multivariate statistical analysis of the multidimensional luminescence dataset allows both the identification of distinct spectral features in the emission and the mapping of their spatial distribution. These results show a 490 nm peak emitted from the cube faces, with shorter wavelength luminescence coming from the vertices and edges; this provides direct experimental confirmation of theoretical predictions.

We have studied the growth of wurtzite GaN and AlxGa1-xN layers and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced 2 in diameter wurtzite AlxGa1-xN layers up to 10 [mu]m in thickness. Undoped wurtzite AlxGa1-xN films were grown on GaAs (1 1 1)B substrates by a plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The fact that free-standing ternary AlxGa1-xN wafers can be grown is very significant for the potential future production of wurtzite AlxGa1-xN substrates optimized for AlGaN-based device structures.

@Article{strathprints30548,
author = {S.V. Novikov and C.R. Staddon and R.E.L. Powell and A.V. Akimov and F. Luckert and P.R. Edwards and R.W. Martin and A.J. Kent and C.T. Foxon},
title = {Wurtzite {AlₓGa₁₋ₓN} bulk crystals grown by molecular beam epitaxy},
journal = {Journal of Crystal Growth},
year = {2011},
volume = {322},
number = {1},
pages = {23--26},
month = {May},
abstract = {We have studied the growth of wurtzite GaN and AlxGa1-xN layers and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced 2 in diameter wurtzite AlxGa1-xN layers up to 10 [mu]m in thickness. Undoped wurtzite AlxGa1-xN films were grown on GaAs (1 1 1)B substrates by a plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The fact that free-standing ternary AlxGa1-xN wafers can be grown is very significant for the potential future production of wurtzite AlxGa1-xN substrates optimized for AlGaN-based device structures.},
keywords = {semiconducting III-V materials, substrates, molecular beam epitaxy, nitrides, wurtzite, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/30548/}
}

InGaN/GaN multiple quantum wells (MQWs) have been studied by using cathodoluminescence hyperspectral imaging with high spatial resolution. Variations in peak emission energies and intensities across trenchlike features and V-pits on the surface of the MQWs are investigated. The MQW emission from the region inside trenchlike features is redshifted by approximately 45 meV and more intense than the surrounding planar regions of the sample, whereas emission from the V-pits is blueshifted by about 20 meV and relatively weaker. By employing this technique to the studied nanostructures it is possible to investigate energy and intensity shifts on a 10 nm length scale.

@Article{strathprints30547,
author = {Jochen Bruckbauer and Paul R. Edwards and Tao Wang and Robert W. Martin},
title = {High resolution cathodoluminescence hyperspectral imaging of surface features in InGaN/GaN multiple quantum well structures},
journal = {Applied Physics Letters},
year = {2011},
volume = {98},
number = {14},
pages = {141908},
month = {April},
abstract = {InGaN/GaN multiple quantum wells (MQWs) have been studied by using cathodoluminescence hyperspectral imaging with high spatial resolution. Variations in peak emission energies and intensities across trenchlike features and V-pits on the surface of the MQWs are investigated. The MQW emission from the region inside trenchlike features is redshifted by approximately 45 meV and more intense than the surrounding planar regions of the sample, whereas emission from the V-pits is blueshifted by about 20 meV and relatively weaker. By employing this technique to the studied nanostructures it is possible to investigate energy and intensity shifts on a 10 nm length scale.},
keywords = {cathodoluminescence, gallium compounds, III-V semiconductors, indium compounds, MOCVD, nanofabrication, nanostructured materials, red shift, semiconductor growth, semiconductor quantum wells, wide band gap semiconductors, Physics, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/30547/}
}

We give an overview of the use of cathodoluminescence (CL) in scanning electron microscopy (SEM) for the nano-scale characterization of semiconducting materials and devices. We discuss the technical aspects of the measurement, such as factors limiting the spatial resolution and design considerations for efficient collection optics. The advantages of more recent developments in the technique are outlined, including the use of the hyperspectral imaging mode and the combination of CL and other SEM-based measurements. We illustrate these points with examples from our own experience of designing and constructing CL systems and applying the technique to the characterization of III-nitride materials and nanostructures.

@article{strathprints30546,
volume = {26},
number = {6},
month = {June},
author = {Paul R Edwards and Robert W Martin},
title = {Cathodoluminescence nano-characterization of semiconductors},
journal = {Semiconductor Science and Technology},
pages = {064005},
year = {2011},
keywords = {optics, condensed matter, electrical matter, magnetic matter, thin films, cathodoluminescence, ionoluminescence , Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/30546/},
abstract = {We give an overview of the use of cathodoluminescence (CL) in scanning electron microscopy (SEM) for the nano-scale characterization of semiconducting materials and devices. We discuss the technical aspects of the measurement, such as factors limiting the spatial resolution and design considerations for efficient collection optics. The advantages of more recent developments in the technique are outlined, including the use of the hyperspectral imaging mode and the combination of CL and other SEM-based measurements. We illustrate these points with examples from our own experience of designing and constructing CL systems and applying the technique to the characterization of III-nitride materials and nanostructures.}
}

The chemical composition of Cu(In,Ga)Se-2 (CIGS) semiconductor compounds is analyzed by local x-ray spectral microanalysis and scanning Auger electron spectroscopy. X-ray diffraction analysis reveals a difference in the predominant orientation of CIGS films depending on the technological conditions under which they are grown. The chemical composition is found to have a strong effect on the shift in the self-absorption edge of CIGS compounds. It is shown that a change in the relative proportion of Ga and In in CIGS semiconducting compounds leads to a change in the band gap E-g for this material in the 1.05-1.72 eV spectral range at 4.2 K.

@Article{strathprints35401,
author = {A. V. Mudryi and V. F. Gremenok and A. V. Karotki and V. B. Zalesski and M. V. Yakushev and F. Luckert and R. Martin},
title = {Structural and optical properties of thin films of Cu(In,Ga)Se₂ semiconductor compounds},
journal = {Journal of Applied Spectroscopy},
year = {2010},
volume = {77},
number = {3},
pages = {371--377},
month = {July},
abstract = {The chemical composition of Cu(In,Ga)Se-2 (CIGS) semiconductor compounds is analyzed by local x-ray spectral microanalysis and scanning Auger electron spectroscopy. X-ray diffraction analysis reveals a difference in the predominant orientation of CIGS films depending on the technological conditions under which they are grown. The chemical composition is found to have a strong effect on the shift in the self-absorption edge of CIGS compounds. It is shown that a change in the relative proportion of Ga and In in CIGS semiconducting compounds leads to a change in the band gap E-g for this material in the 1.05-1.72 eV spectral range at 4.2 K.},
keywords = {chalcopyrite semiconductors, chemical composition, absorption coefficient, x-ray diffraction analysis, structure, CUINSE2 SINGLE-CRYSTALS, SOLAR-CELLS, EPITAXIAL LAYERS, CUIN1-XGAXSE2, QUALITY, PHOTOREFLECTANCE, PERFORMANCE, EFFICIENCY, Spectroscopy, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/35401/}
}

Study of the luminescence of Rare Earth ions introduced into III-nitride semiconductor hosts produces a wealth of important information. This chapter describes investigations of photoluminescence and cathodoluminescence spectroscopy on a range of RE-implanted samples, with hosts including GaN, AlGaN and AlInN. Raising the post-implantation annealing temperature is shown to lead to a dramatic increase in RE luminescence intensity, and methods to allow annealing well above the growth temperature of the host are discussed. The high-brightness samples that result enable the resolution of additional fine-structure in the luminescence from GaN:Eu and clarification of multiple sites for the RE. Measurements for AlGaN hosts covering the entire composition range point to the importance of core-excitons in the luminescence process. Adding in information from X-ray microanalysis and high spatial resolution luminescence mapping reveals further details of the effects resulting from annealing and of changes in host composition.

@incollection{strathprints35400,
author = {Robert Martin},
series = {Topics in Applied Physics},
booktitle = {Rare Earth Doped III-Nitrides For Optoelectronic And Spintronic Applications},
editor = {Kevin Peter O'Donnell and Volkmar Dierolf},
address = {Berlin},
title = {Microscopic characterisation of luminescent III-N:RE epilayers},
publisher = {Springer-Verlag Berlin},
year = {2010},
pages = {189--219},
keywords = {rare earth ions, eu-implanted gan, electroluminescent Devices, optical-properties, light emission, blue emission, III-Nitrides, thin films, photoluminescence, ER, Physics},
url = {http://strathprints.strath.ac.uk/35400/},
abstract = {Study of the luminescence of Rare Earth ions introduced into III-nitride semiconductor hosts produces a wealth of important information. This chapter describes investigations of photoluminescence and cathodoluminescence spectroscopy on a range of RE-implanted samples, with hosts including GaN, AlGaN and AlInN. Raising the post-implantation annealing temperature is shown to lead to a dramatic increase in RE luminescence intensity, and methods to allow annealing well above the growth temperature of the host are discussed. The high-brightness samples that result enable the resolution of additional fine-structure in the luminescence from GaN:Eu and clarification of multiple sites for the RE. Measurements for AlGaN hosts covering the entire composition range point to the importance of core-excitons in the luminescence process. Adding in information from X-ray microanalysis and high spatial resolution luminescence mapping reveals further details of the effects resulting from annealing and of changes in host composition.}
}

DFTB+ is a recent general purpose implementation of density-functional based tight binding. One of the early motivators to develop this code was to investigate lanthanide impurities in nitride semiconductors, leading to a series of successful studies into structure and electrical properties of these systems. Here we describe our general framework to treat the physical effects needed for these problematic impurities within a tight-binding formalism, additionally discussing forces and stresses in DFTB. We also present an approach to evaluate the general case of Slater-Koster transforms and all of their derivatives in Cartesian coordinates. These developments are illustrated by simulating isolated Gd impurities in GaN.

@Article{strathprints34306,
author = {B. Hourahine and B Aradi and T. Frauenheim},
title = {DFTB+ and lanthanides},
journal = {Journal of Physics Conference Series},
year = {2010},
volume = {242},
number = {1},
pages = {012005},
abstract = {DFTB+ is a recent general purpose implementation of density-functional based tight binding. One of the early motivators to develop this code was to investigate lanthanide impurities in nitride semiconductors, leading to a series of successful studies into structure and electrical properties of these systems. Here we describe our general framework to treat the physical effects needed for these problematic impurities within a tight-binding formalism, additionally discussing forces and stresses in DFTB. We also present an approach to evaluate the general case of Slater-Koster transforms and all of their derivatives in Cartesian coordinates. These developments are illustrated by simulating isolated Gd impurities in GaN.},
keywords = {crystals, semiconductors, crystal impurities, condensed matter, tight-binding approach, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/34306/}
}

Thin films of Cu(In, Ga)Se-2 (CIGS) with a Ga/(Ga + In) ratio of similar to 0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (E-g) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P-1 at similar to 0.91 eV and P-2 at similar to 0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu-In) and indium vacancies V-In, respectively, as the simplest radiation-induced defects.

@Article{strathprints32495,
author = {A. V. Karotki and A.V. Mudryi and Michael V Yakushev and Franziska Luckert and Robert Martin},
title = {Structural and optical properties of CdS/Cu(In,Ga)Se₂ heterostructures irradiated by high energy electrons},
journal = {Journal of Applied Spectroscopy},
year = {2010},
volume = {77},
number = {5},
pages = {668--674},
month = {November},
abstract = {Thin films of Cu(In, Ga)Se-2 (CIGS) with a Ga/(Ga + In) ratio of similar to 0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (E-g) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P-1 at similar to 0.91 eV and P-2 at similar to 0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu-In) and indium vacancies V-In, respectively, as the simplest radiation-induced defects.},
keywords = {Cu(In,Ga)Se2 , photoluminescence, electron irradiation, defects, Therapeutics. Pharmacology, Spectroscopy, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/32495/}
}

This book provides a snapshot of recent progress in the field of rare-earth-doped group III-nitride semiconductors, especially GaN, but extending to AlN and the alloys AlGaN, AlInN and InGaN. This material class is currently enjoying an upsurge in interest due to its ideal suitability for both optoelectronic and spintronic applications. The text first introduces the reader to the historical background and the major theoretical challenges presented when 4f electron systems are embedded in a semiconductor matrix. It details the preparation of samples for experimental study, either by in-situ growth or ion implantation/annealing, and describes their microscopic structural characterisation. Optical spectroscopy is a dominant theme, complicated by site multiplicity, whether in homogeneous hosts or in heterostructures such as quantum dots, and enlivened by the abiding fascination of the energy transfer mechanism between the host material and the lumophore. Finally, the rapid progress towards prospective optoelectronic and spintronic devices is presented along with several examples

@incollection{strathprints30174,
author = {Kevin O'Donnell},
series = {Topics in applied physics},
booktitle = {Rare-earth doped III-nitrides for optoelectronic and spintronic applications},
editor = {Kevin O'Donnell and Volkmar Dierolf},
title = {Summary and prospects for future work},
publisher = {Springer},
pages = {343--345},
year = {2010},
keywords = {optical \& plasma physics , electronic materials, atomic, molecular, Physics},
url = {http://strathprints.strath.ac.uk/30174/},
abstract = {This book provides a snapshot of recent progress in the field of rare-earth-doped group III-nitride semiconductors, especially GaN, but extending to AlN and the alloys AlGaN, AlInN and InGaN. This material class is currently enjoying an upsurge in interest due to its ideal suitability for both optoelectronic and spintronic applications. The text first introduces the reader to the historical background and the major theoretical challenges presented when 4f electron systems are embedded in a semiconductor matrix. It details the preparation of samples for experimental study, either by in-situ growth or ion implantation/annealing, and describes their microscopic structural characterisation. Optical spectroscopy is a dominant theme, complicated by site multiplicity, whether in homogeneous hosts or in heterostructures such as quantum dots, and enlivened by the abiding fascination of the energy transfer mechanism between the host material and the lumophore. Finally, the rapid progress towards prospective optoelectronic and spintronic devices is presented along with several examples}
}

Eu-doped GaN was grown by organometallic vapor phase epitaxy at temperatures from 900 to 1100 degrees C. Eu incorporation is influenced by temperature with the highest concentration found for growth at 1000 degrees C. In all samples, Eu is incorporated entirely on substitutional Ga sites with a slight displacement which is highest (similar to 0.2 angstrom) in the sample grown at 900 degrees C and mainly directed along the c-axis. The major optical Eu3+ centers are identical for in situ doped and ion-implanted samples after high temperature and pressure annealing. The dominant Eu3+ luminescence lines are attributed to isolated, substitutional Eu. (c) 2010 American Institute of Physics. [doi:10.1063/1.3489103]

We review theoretical investigations into the structure and electrical activity of rare earth (RE) dopants in III-V semiconductors especially GaN. Substitutional rare earth dopants in GaN are found to be electrically inactive and require another defect to enable them to act as strong exciton traps. In contrast AlN is distinctive as it possesses a deep donor level. The electronic structure of complexes of the RE with other defects is discussed along with implications for efficient room temperature luminescence.

High quality Al1-xInxN/GaN bilayers, grown by metal organic chemical vapor deposition (MOCVD), were characterized using structural and optical techniques. Compositional analysis was performed using Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA). The InN molar fraction x decreased approximately linearly with increasing growth temperature and ranged from x = 0.13 to 0.24. Up to x = 0.20 the layers grow pseudomorphically to GaN with good crystalline quality. These layers show a smooth surface with V-shaped pits. Two layers with InN contents around 24\% showed partial strain relaxation. However, the mechanisms leading to relaxation of compressive strain are very different in the two samples grown both at similar temperature but with different growth rates. One sample shows a decreased c/a ratio, as expected for relaxation of the compressive strain, while In was shown to be homogeneously distributed with depth. The other sample started to grow with x = 0.24 but relaxed mainly by reduction of the incorporated InN content towards the lattice-match composition of x similar to 0.17. Both samples have an increased surface roughness. All samples show strong Al1-xInxN band edge luminescence with large bowing parameter and Stokes’ shifts. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

@Article{strathprints29009,
author = {K. Lorenz and S. Magalhaes and N. Franco and N. P. Barradas and V. Darakchieva and E. Alves and S. Pereira and M. R. Correia and F. Munnik and R. W. Martin and K. P. O'Donnell and I. M. Watson},
title = {{AlₓIn₁₋ₓN/GaN} bilayers: Structure, morphology, and optical properties},
journal = {Physica Status Solidi B},
year = {2010},
volume = {247},
number = {7},
pages = {1740--1746},
month = {July},
abstract = {High quality Al1-xInxN/GaN bilayers, grown by metal organic chemical vapor deposition (MOCVD), were characterized using structural and optical techniques. Compositional analysis was performed using Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA). The InN molar fraction x decreased approximately linearly with increasing growth temperature and ranged from x = 0.13 to 0.24. Up to x = 0.20 the layers grow pseudomorphically to GaN with good crystalline quality. These layers show a smooth surface with V-shaped pits. Two layers with InN contents around 24\% showed partial strain relaxation. However, the mechanisms leading to relaxation of compressive strain are very different in the two samples grown both at similar temperature but with different growth rates. One sample shows a decreased c/a ratio, as expected for relaxation of the compressive strain, while In was shown to be homogeneously distributed with depth. The other sample started to grow with x = 0.24 but relaxed mainly by reduction of the incorporated InN content towards the lattice-match composition of x similar to 0.17. Both samples have an increased surface roughness. All samples show strong Al1-xInxN band edge luminescence with large bowing parameter and Stokes' shifts. (C) 2010 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
keywords = {photoluminescence, Rutherford backscattering spectrometry, surface structure, III-V semiconductors, field-effect transistor, alinn, alloys, growth, layers, movpe, GaN, optics, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/29009/}
}

High-quality CuInSe2 single crystals were studied using polarization resolved photoluminescence (PL) and magnetophotoluminescence (MPL). The emission lines related to the first (n=2) excited states for the A and B free excitons were observed in the PL and MPL spectra at 1.0481 meV and 1.0516 meV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 meV and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (E-g(A)=1.050 eV and E-g(B)=1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.

Single crystals of CuGaSe2 were studied using magnetophotoluminescence inmagnetic fields up to 20 T at 4.2 K. The rate of the diamagnetic shift in the A free exciton peak was determined to be 9.82 x 10(-6) eV/T-2. This rate was used to calculate the reduced mass as 0.115m(0), the binding energy as 12.9 meV, the Bohr radius as 5.1 nm and an effective hole mass of 0.64m(0) (m(0) is the free electron mass) of the free A exciton using a low-field perturbation approach and the hydrogenic model.

@Article{strathprints29007,
author = {F. Luckert and M. V. Yakushev and C. Faugeras and A. V. Karotki and A. V. Mudryi and R. W. Martin},
title = {Diamagnetic shift of the A free exciton in CuGaSe₂ single crystals},
journal = {Applied Physics Letters},
year = {2010},
volume = {97},
number = {16},
pages = {162101},
month = {October},
abstract = {Single crystals of CuGaSe2 were studied using magnetophotoluminescence inmagnetic fields up to 20 T at 4.2 K. The rate of the diamagnetic shift in the A free exciton peak was determined to be 9.82 x 10(-6) eV/T-2. This rate was used to calculate the reduced mass as 0.115m(0), the binding energy as 12.9 meV, the Bohr radius as 5.1 nm and an effective hole mass of 0.64m(0) (m(0) is the free electron mass) of the free A exciton using a low-field perturbation approach and the hydrogenic model.},
keywords = {electical properties, thin films, cuinse2, crystals, Physics, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/29007/}
}

Two routes for the fabrication of bulk GaN microcavities embedded between two dielectric mirrors are described, and the optical properties of the microcavities thus obtained are compared. In both cases, the GaN active layer is grown by molecular beam epitaxy on (111) Si, allowing use of selective etching to remove the substrate. In the first case, a three period Al0.2Ga0.8N / AlN Bragg mirror followed by a lambda/2 GaN cavity are grown directly on the Si. In the second case, a crack-free 2,mu m thick GaN layer is grown, and progressively thinned to a final thickness of lambda. Both devices work in the strong coupling regime at low temperature, as evidenced by angle-dependent reflectivity or transmission experiments. However, strong light-matter coupling in emission at room temperature is observed only for the second one. This is related to the poor optoelectronic quality of the active layer of the first device, due to its growth only 250 nm above the Si substrate and its related high defect density. The reflectivity spectra of the microcavities are well accounted for by using transfer matrix calculations. (C) 2010 American Institute of Physics. [doi:10.1063/1.3477450]

@Article{strathprints28963,
author = {F. Reveret and K. Bejtka and P. R. Edwards and S. Chenot and I. R. Sellers and P. Disseix and A. Vasson and J. Leymarie and J. Y. Duboz and M. Leroux and F. Semond and Robert Martin},
title = {Strong light-matter coupling in bulk GaN-microcavities with double dielectric mirrors fabricated by two different methods},
journal = {Journal of Applied Physics},
year = {2010},
volume = {108},
number = {4},
pages = {043524},
month = {August},
abstract = {Two routes for the fabrication of bulk GaN microcavities embedded between two dielectric mirrors are described, and the optical properties of the microcavities thus obtained are compared. In both cases, the GaN active layer is grown by molecular beam epitaxy on (111) Si, allowing use of selective etching to remove the substrate. In the first case, a three period Al0.2Ga0.8N / AlN Bragg mirror followed by a lambda/2 GaN cavity are grown directly on the Si. In the second case, a crack-free 2,mu m thick GaN layer is grown, and progressively thinned to a final thickness of lambda. Both devices work in the strong coupling regime at low temperature, as evidenced by angle-dependent reflectivity or transmission experiments. However, strong light-matter coupling in emission at room temperature is observed only for the second one. This is related to the poor optoelectronic quality of the active layer of the first device, due to its growth only 250 nm above the Si substrate and its related high defect density. The reflectivity spectra of the microcavities are well accounted for by using transfer matrix calculations. (C) 2010 American Institute of Physics. [doi:10.1063/1.3477450]},
keywords = {semiconductor microcavities, laser, aluminium compounds, distributed Bragg reflectors, gallium compounds, molecular beam epitaxial growth, photoluminescence, semiconductor growth , wide band gap semiconductors, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/28963/}
}

Microstructures containing GaN/air distributed Bragg reflector (DBR) regions were fabricated by a selective wet etch to remove sacrificial AlInN layers from GaN-AlInN multilayers. The epitaxial multilayers were grown on free-standing GaN substrates, and contained AlInN essentially lattice matched with GaN in order to minimize strain. Two geometries were defined for study by standard lithographic techniques and dry etching: cylindrical pillars and doubly anchored rectangular bridges. Microreflectivity spectra were recorded from the air-gap DBRs, and indicated peak reflectivities exceeding 70\% for a typical 3-period microbridge. These values are likely to be limited by the small scale of the features in comparison with the measurement spot. The stopband in this case was centred at 409 nm, and the reflectivity exceeded 90\% of the maximum over 73 nm. Simulations of reflectance spectra, including iterations to layer thicknesses, gave insight into the tolerances achievable in processing, in particular indicating bounds on the parasitic removal of GaN layers during wet etching. Air-gap nitride DBRs as described can be further developed in various ways, including adaptation for electrostatic tuning, incorporation into microcavities, and integration with active emitters.

@Article{strathprints27666,
author = {C. Xiong and P.R. Edwards and G. Christmann and E. Gu and Martin Dawson and J.J. Baumberg and R.W. Martin and I.M. Watson},
title = {High reflectivity GaN/Air vertical distributed Bragg reflectors fabricated by wet etching of sacrificial AllnN layers},
journal = {Semiconductor Science and Technology},
year = {2010},
volume = {25},
number = {3},
pages = {032001},
month = {February},
abstract = {Microstructures containing GaN/air distributed Bragg reflector (DBR) regions were fabricated by a selective wet etch to remove sacrificial AlInN layers from GaN-AlInN multilayers. The epitaxial multilayers were grown on free-standing GaN substrates, and contained AlInN essentially lattice matched with GaN in order to minimize strain. Two geometries were defined for study by standard lithographic techniques and dry etching: cylindrical pillars and doubly anchored rectangular bridges. Microreflectivity spectra were recorded from the air-gap DBRs, and indicated peak reflectivities exceeding 70\% for a typical 3-period microbridge. These values are likely to be limited by the small scale of the features in comparison with the measurement spot. The stopband in this case was centred at 409 nm, and the reflectivity exceeded 90\% of the maximum over 73 nm. Simulations of reflectance spectra, including iterations to layer thicknesses, gave insight into the tolerances achievable in processing, in particular indicating bounds on the parasitic removal of GaN layers during wet etching. Air-gap nitride DBRs as described can be further developed in various ways, including adaptation for electrostatic tuning, incorporation into microcavities, and integration with active emitters.},
keywords = {GaN/Air, Bragg reflectors, wet etching, AllnN layers, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/27666/}
}

We identify a dominant light-emitting center in ion-implanted GaN:Eu3+ for which the lattice damage has been completely healed, according to x-ray diffraction and Rutherford backscattering spectrometry measurements, by high-temperature, high-pressure annealing. This center is likely to be the isolated substitutional EuGa defect. It lacks a ‘subgap’ excitation band and therefore has no state in the GaN band gap, shows threefold splitting of its 7F2 level, with two sublevels nearly degenerate, and exhibits a long, single-exponential luminescence decay. Competing luminescent centers of GaN:Eu involve this prime center with intrinsic lattice defects, one of which may also be responsible for the GaN yellow band.

@Article{strathprints26804,
author = {I.S. Roqan and K.P. O'Donnell and R.W. Martin and P.R. Edwards and S.F. Song and A. Vantomme and K. Lorenz and E. Alves and M. Bo{\'c}kowski},
title = {Identification of the prime optical center in GaN:Eu³⁺},
journal = {Physical Review B},
year = {2010},
volume = {81},
number = {1},
pages = {085209},
month = {February},
abstract = {We identify a dominant light-emitting center in ion-implanted GaN:Eu3+ for which the lattice damage has been completely healed, according to x-ray diffraction and Rutherford backscattering spectrometry measurements, by high-temperature, high-pressure annealing. This center is likely to be the isolated substitutional EuGa defect. It lacks a 'subgap' excitation band and therefore has no state in the GaN band gap, shows threefold splitting of its 7F2 level, with two sublevels nearly degenerate, and exhibits a long, single-exponential luminescence decay. Competing luminescent centers of GaN:Eu involve this prime center with intrinsic lattice defects, one of which may also be responsible for the GaN yellow band.},
keywords = {optics, GaN, Eu3+, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/26804/}
}

The platinum-based anticancer drugs cisplatin, carboplatin, and oxaliplatin are an important component of chemotherapy but are limited by severe dose-limiting side effects and the ability of tumors to develop resistance rapidly. These drugs can be improved through the use of drug-delivery vehicles that are able to target cancers passively or actively. In this study, we have tethered the active component of the anticancer drug oxaliplatin to a gold nanoparticle for improved drug delivery. Naked gold nanoparticles were functionalized with a thiolated poly(ethylene glycol) (PEG) monolayer capped with a carboxylate group. [Pt(1R,2R-diaminocyclohexane)(H2O)2]2NO3 was added to the PEG surface to yield a supramolecular complex with 280 ({$\pm$}20) drug molecules per nanoparticle. The platinum-tethered nanoparticles were examined for cytotoxicity, drug uptake, and localization in the A549 lung epithelial cancer cell line and the colon cancer cell lines HCT116, HCT15, HT29, and RKO. The platinum-tethered nanoparticles demonstrated as good as, or significantly better, cytotoxicity than oxaliplatin alone in all of the cell lines and an unusual ability to penetrate the nucleus in the lung cancer cells.

@article{strathprints26140,
volume = {132},
number = {13},
month = {April},
author = {Sarah D. Brown and Paola Nativo and Jo-Ann Smith and David Stirling and Paul R. Edwards and Balaji Venugopal and David J. Flint and Jane A. Plumb and Duncan Graham and Nial J. Wheate},
title = {Gold nanoparticles for the improved anticancer drug delivery of the active component of oxaliplatin},
journal = {Journal of the American Chemical Society},
pages = {4678--4684},
year = {2010},
keywords = {cancer therapy, complexes, cells, cytotoxicity, chemotherapy, conjugate, stability, vectors, system, folate, Therapeutics. Pharmacology, Physics, Biochemistry, Colloid and Surface Chemistry, Chemistry(all), Catalysis},
url = {http://strathprints.strath.ac.uk/26140/},
abstract = {The platinum-based anticancer drugs cisplatin, carboplatin, and oxaliplatin are an important component of chemotherapy but are limited by severe dose-limiting side effects and the ability of tumors to develop resistance rapidly. These drugs can be improved through the use of drug-delivery vehicles that are able to target cancers passively or actively. In this study, we have tethered the active component of the anticancer drug oxaliplatin to a gold nanoparticle for improved drug delivery. Naked gold nanoparticles were functionalized with a thiolated poly(ethylene glycol) (PEG) monolayer capped with a carboxylate group. [Pt(1R,2R-diaminocyclohexane)(H2O)2]2NO3 was added to the PEG surface to yield a supramolecular complex with 280 ({$\pm$}20) drug molecules per nanoparticle. The platinum-tethered nanoparticles were examined for cytotoxicity, drug uptake, and localization in the A549 lung epithelial cancer cell line and the colon cancer cell lines HCT116, HCT15, HT29, and RKO. The platinum-tethered nanoparticles demonstrated as good as, or significantly better, cytotoxicity than oxaliplatin alone in all of the cell lines and an unusual ability to penetrate the nucleus in the lung cancer cells.}
}

A novel epitaxial lift-off process for III-nitrides, involving selective removal of a sacrificial (Al, In)N layer in a hot nitric acid etchant, is reported. This was applied to the fabrication of 1?{\ensuremath{\lambda}} GaN planar microcavities bounded by two dielectric DBRs, starting from epitaxial GaN-(Al, In)N-GaN trilayers grown on free-standing GaN or high-quality GaN template material. An optically smooth surface was retained on the GaN surface exposed to the nitric acid etch, with root mean square roughness values as low as 2 nm over 8 {\ensuremath{\mu}}m{$\times$}8 {\ensuremath{\mu}}m areas. Photoluminescence and reflectivity spectra were recorded from completed microcavities, and the latter showed clear dips in the region of 3.5 eV.

The effects of magnetic fields up to 20 T were studied in CuInSe2 single crystals using photoluminescence (PL) at 4.2 K. Diamagnetic shifts of the free A and B excitons measured in the PL spectra in CuInSe2 at 4.2 K tinder the magnetic fields were used to estimate the reduced masses (0.095m(0) for the A and 0.098m(0) for the B exciton), binding energies (7.0 meV for the A and 7.2 meV for the B exciton) and Bohr radii (7.6 nm for the A and 7.3 nm for the B exciton) of the free-excitons in CuInSe2 assuming that both excitons are isotropic and hydrogen-like. (C) 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim

@Article{strathprints37429,
author = {M. V. Yakushev and R. W. Martin and A. Babinski and A. V. Mudryi},
title = {Effects of magnetic fields on free excitons in CuInSe₂},
journal = {Physica Status Solidi C},
year = {2009},
volume = {6},
number = {5},
pages = {1086--1088},
month = {February},
note = {Special Issue: 16th International Conference on Ternary and Multinary Compounds (ICTMC16)},
abstract = {The effects of magnetic fields up to 20 T were studied in CuInSe2 single crystals using photoluminescence (PL) at 4.2 K. Diamagnetic shifts of the free A and B excitons measured in the PL spectra in CuInSe2 at 4.2 K tinder the magnetic fields were used to estimate the reduced masses (0.095m(0) for the A and 0.098m(0) for the B exciton), binding energies (7.0 meV for the A and 7.2 meV for the B exciton) and Bohr radii (7.6 nm for the A and 7.3 nm for the B exciton) of the free-excitons in CuInSe2 assuming that both excitons are isotropic and hydrogen-like. (C) 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim},
editor = {S Sadewasser and D AbouRas and B Lake and HW Schock},
keywords = {optical-properties, energy, constant, single-crystals, Physics, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/37429/}
}

Radiative recombination processes in CuInSe2 (CIS) single crystals grown by the, vertical Bridgman technique were studied using photoluminescence (PL) and reflectance (RF) spectroscopies at temperatures from 4.2 to 60 K, and excitation intensity from 0.6 to 30 W/cm(2). Study of the quenching parameters of the A and B freeo and first three bound-excitons (M1, M2 and M3) in high-quality CuInSe2 single crystal leads to estimates for the binding energy of the A (7.7 meV) and B (7.9 meV) free excitons as well as dissociation energies for the M1, M2 and M3 bound-excitons.

InxGa1-xN/GaN quantum wells have been grown on the \{1011\} facets of dense arrays of self-assembled GaN nano-pyramids formed by selective area growth and characterised by high spatial resolution cathodoluminescence. The pyramids are shown to have significantly reduced defect (green-yellow) band emission and the quantum well luminescence is correspondingly intense. The peak energy of this luminescence is shown to blue-shift as the sampled region is moved up the pyramid facets, revealing that InN incorporation in such closely spaced epitaxial nanostructures differs from that in widely spaced micron-size pyramidal structures decreasing rather than increasing towards the nano-pyramid tips.

@article{strathprints26816,
volume = {2},
month = {December},
title = {Light emission from InGaN quantum wells grown on the facets of closely spaced GaN nano-pyramids formed by nano-imprinting},
author = {Chaowang Liu and Alexander Satka and L.K. Jagadamma and P.R. Edwards and D. Allsopp and R.W. Martin and Philip Shields and Jaroslav Kovac and Frantisek Uherek and Wang Wang},
year = {2009},
pages = {121002},
journal = {Applied Physics Express},
keywords = {light emission, luminescence , quantum wells, cathodoluminescence, Physics, Physics and Astronomy(all), Engineering(all)},
url = {http://strathprints.strath.ac.uk/26816/},
abstract = {InxGa1-xN/GaN quantum wells have been grown on the \{1011\} facets of dense arrays of self-assembled GaN nano-pyramids formed by selective area growth and characterised by high spatial resolution cathodoluminescence. The pyramids are shown to have significantly reduced defect (green-yellow) band emission and the quantum well luminescence is correspondingly intense. The peak energy of this luminescence is shown to blue-shift as the sampled region is moved up the pyramid facets, revealing that InN incorporation in such closely spaced epitaxial nanostructures differs from that in widely spaced micron-size pyramidal structures decreasing rather than increasing towards the nano-pyramid tips.}
}

Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1?xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17{\ensuremath{<}}x{\ensuremath{<}}0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition, and sharp, well defined optical absorption edges. The band gap energy varies in a broad energy range from {$\sim$} 3.4 eV in GaN to {$\sim$} 0.8 eV at x {$\sim$} 0.85. The reduction in the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x{\ensuremath{>}}0.2, and to the upward movement of the valence band for alloys with x{\ensuremath{<}}0.2. The unique features of the band structure offer an opportunity of using GaN1?xAsx alloys for various types of solar power conversion devices.

@Article{strathprints26799,
author = {K.M. Yu and S.V. Novikov and R. Broesler and I.N. Demchenko and F. Luckert and R.W. Martin and J.D. Denlinger and Z. Liliental-Weber and W. Walukiewicz and C.T. Foxon},
title = {Highly mismatched crystalline and amorphous GaN₁₋ₓAsₓ alloys in the whole composition range},
journal = {Journal of Applied Physics},
year = {2009},
volume = {106},
number = {10},
pages = {103709},
month = {November},
abstract = {Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1?xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17{\ensuremath{<}}x{\ensuremath{<}}0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition, and sharp, well defined optical absorption edges. The band gap energy varies in a broad energy range from {$\sim$} 3.4 eV in GaN to {$\sim$} 0.8 eV at x {$\sim$} 0.85. The reduction in the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x{\ensuremath{>}}0.2, and to the upward movement of the valence band for alloys with x{\ensuremath{<}}0.2. The unique features of the band structure offer an opportunity of using GaN1?xAsx alloys for various types of solar power conversion devices.},
keywords = {alloying, amorphous semiconductors, conduction bands, electronegativity, energy gap, gallium arsenide, molecular beam epitaxial growth, nitrogen compounds, solar energy conversion, solubility, valence bands, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/26799/}
}

We demonstrate nanoparticle size measurement using time-resolved fluorescence anisotropy decay in relation to establishing a nanometrology standard. The rotational correlation time equivalent to the isotropic Brownian rotation of a fluorescent 6-methoxyquinolinium dye attached to amorphous silica nanoparticles was determined in three different LUDOX* colloids from the complex fluorescence anisotropy decay observed. Once competing depolarization and nanoparticle aggregation had been taken into account, good agreement was found of 4.0 {$\pm$} 0.4 nm, 6.4 {$\pm$} 0.5 nm and 11.0 {$\pm$} 1.6 nm corresponding to the manufacturer’s reported particle radii of 3.5 nm, 6 nm and 11 nm, for LUDOX SM30, AM30 and AS40 respectively. We describe the measurement science required for acquisition and interpretation of fluorescence anisotropy decay data in order to determine nanoparticle size while highlighting the limitations and useful range of measurement.

Single crystals of CuInS2 were studied with reflectance (RF) and photoluminescence (PL) at 4.2 K in magnetic fields up to 10 T. The diamagnetic energy shifts observed for the free excitonic lines in both the RF and PL spectra were used to calculate the reduced masses (0.141m0, 0.142m0, and 0.114m0), binding energies (18.46, 18.59, and 14.90 meV), Bohr radii (3.82, 3.79, and 4.73 nm), and hole effective masses (1.20m0, 1.28m0, and 0.40m0) for the ALPB, AUPB, and BC excitons, respectively, using a low-field perturbation approach.

Thin films of Cu(In,Ga)Se2 (CIGS) were deposited at temperatures below 450 ?C on polyimide (PI) substrates coated with Mo in a roll-to-roll set up by a combination of co-evaporation and ion-beam techniques. Flexible solar cells ITO/i-ZnO/CdS/CIGS/Mo/PI with and without Na incorporation were then fabricated. The films and solar cells were examined by: X-ray fluorescence spectroscopy (XRF) and Auger electron spectroscopy (AES), to determine the elemental composition, as well as by X-ray diffraction for structure- and scanning electron microscopy (SEM) for morphology-analysis. Photoluminescence (PL) and PL-excitation (PLE) at temperatures from 4.2 to 78 K were also used to estimate the band-gap energy of CIGS, examine the electronic properties and defect nature. The aim of this study was to correlate the incorporation of Na with optical and structural parameters of the CIGS layers as well as with the solar cell performance.

@Article{strathprints19251,
author = {H. Zachmann and S. Heinker and A. Braun and A.V. Mudryi and V.F. Gremenok and A.V. Ivaniukovich and M.V. Yakushev},
title = {Characterisation of Cu(In,Ga)Se₂-based thin film solar cells on polyimide},
journal = {Thin Solid Films},
year = {2009},
volume = {517},
number = {7},
pages = {2209--2212},
month = {February},
abstract = {Thin films of Cu(In,Ga)Se2 (CIGS) were deposited at temperatures below 450 ?C on polyimide (PI) substrates coated with Mo in a roll-to-roll set up by a combination of co-evaporation and ion-beam techniques. Flexible solar cells ITO/i-ZnO/CdS/CIGS/Mo/PI with and without Na incorporation were then fabricated. The films and solar cells were examined by: X-ray fluorescence spectroscopy (XRF) and Auger electron spectroscopy (AES), to determine the elemental composition, as well as by X-ray diffraction for structure- and scanning electron microscopy (SEM) for morphology-analysis. Photoluminescence (PL) and PL-excitation (PLE) at temperatures from 4.2 to 78 K were also used to estimate the band-gap energy of CIGS, examine the electronic properties and defect nature. The aim of this study was to correlate the incorporation of Na with optical and structural parameters of the CIGS layers as well as with the solar cell performance.},
keywords = {Cu(In, Ga)Se-2, solar cells, polyimide, photoluminescence, characterisation, thin solar cells, Physics, Materials Chemistry, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Surfaces, Coatings and Films, Metals and Alloys},
url = {http://strathprints.strath.ac.uk/19251/}
}

We present a method of simultaneously calculating both the internal and external fields of arbitrarily shaped dielectric and metallic axisymmetric nanoparticles. By using a set of distributed spherical vector wavefunctions that are exact solutions to Maxwell’s equations and which form a complete, linearly independent set on the particle surface, we approximate the surface Green functions of particles. In this way we can enforce the boundary conditions at the interface and represent the electromagnetic fields at the surface to an arbitrary precision. With the boundary conditions at the particle surface satisfied, the electromagnetic fields are uniquely determined at any point in space, whether internal or external to the particle. Furthermore, the residual field error at the particle surface can be shown to give an upper bound error for the field solutions at any point in space. We show the accuracy of this method with two important areas studied widely in the literature, photonic nanojets and the internal field structure of nanoparticles.

@Article{strathprints19220,
author = {K. Holms and B. Hourahine and F. Papoff},
title = {Calculation of internal and scattered fields of axisymmetric nanoparticles at any point in space},
journal = {Journal of Optics A: Pure and Applied Optics},
year = {2009},
volume = {11},
number = {5},
pages = {054009},
month = {May},
abstract = {We present a method of simultaneously calculating both the internal and external fields of arbitrarily shaped dielectric and metallic axisymmetric nanoparticles. By using a set of distributed spherical vector wavefunctions that are exact solutions to Maxwell's equations and which form a complete, linearly independent set on the particle surface, we approximate the surface Green functions of particles. In this way we can enforce the boundary conditions at the interface and represent the electromagnetic fields at the surface to an arbitrary precision. With the boundary conditions at the particle surface satisfied, the electromagnetic fields are uniquely determined at any point in space, whether internal or external to the particle. Furthermore, the residual field error at the particle surface can be shown to give an upper bound error for the field solutions at any point in space. We show the accuracy of this method with two important areas studied widely in the literature, photonic nanojets and the internal field structure of nanoparticles.},
keywords = {scattering, surface green functions, near field, Optics. Light, Atomic and Molecular Physics, and Optics},
url = {http://strathprints.strath.ac.uk/19220/}
}

Epilayers of the quaternary alloy AlxInyGa1?x?yN have been grown on GaN/sapphire templates by plasma-assisted molecular beam epitaxy. The emission properties and elemental compositions of these samples were evaluated simultaneously and intercorrelated by combining hyperspectral cathodoluminescence imaging and wavelength-dispersive X-ray mapping. Use was made of inherent variations in growth temperature across a single epilayer to study the resultant effect on the different metal fractions and luminescence emission wavelength. By examining statistical correlations in this data, the interdependence of the fractions of constituent binary compounds, together with the associated changes in emission characteristics, can be clarified without the need to grow a systematic series of samples.

@article{strathprints19166,
volume = {45},
number = {4-5},
month = {April},
author = {Paul Edwards and R.W. Martin and K. Bejtka and K.P. O'Donnell and S. Fernandez-Garrido and E. Calleja},
title = {Correlating composition and luminescence in AlInGaN epilayers},
journal = {Superlattices and Microstructures},
pages = {151--155},
year = {2009},
keywords = {AlInGaN, GaN, MBE, cathodoluminescence, WDX, Physics, Materials Science(all), Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/19166/},
abstract = {Epilayers of the quaternary alloy AlxInyGa1?x?yN have been grown on GaN/sapphire templates by plasma-assisted molecular beam epitaxy. The emission properties and elemental compositions of these samples were evaluated simultaneously and intercorrelated by combining hyperspectral cathodoluminescence imaging and wavelength-dispersive X-ray mapping. Use was made of inherent variations in growth temperature across a single epilayer to study the resultant effect on the different metal fractions and luminescence emission wavelength. By examining statistical correlations in this data, the interdependence of the fractions of constituent binary compounds, together with the associated changes in emission characteristics, can be clarified without the need to grow a systematic series of samples.}
}

Photoluminescence of CuInS2 single crystals grown by both the traveling heater method (THM) and chemical vapor transport (CVT) has been investigated at 4.2, 78, and 300 K. Intense emission in the near-band-edge region caused by free and bound excitons has been detected for both types of crystals. Taking into account the energy position of the luminescence line of the ground (n = 1) and first excited (n = 2) states, the binding energy for free A excitons has been estimated to be about 19.7 and 18.5 meV for CuInS2 grown by CVT and THM, respectively.

@Article{strathprints19124,
author = {A.V. Mudryi and A.V. Karotki and M.V. Yakushev and R.W. Martin},
title = {Photoluminescence of CuInS₂ single crystals grown by traveling heater and chemical vapor transport methods},
journal = {Journal of Applied Spectroscopy},
year = {2009},
volume = {76},
number = {2},
pages = {215--219},
month = {March},
abstract = {Photoluminescence of CuInS2 single crystals grown by both the traveling heater method (THM) and chemical vapor transport (CVT) has been investigated at 4.2, 78, and 300 K. Intense emission in the near-band-edge region caused by free and bound excitons has been detected for both types of crystals. Taking into account the energy position of the luminescence line of the ground (n = 1) and first excited (n = 2) states, the binding energy for free A excitons has been estimated to be about 19.7 and 18.5 meV for CuInS2 grown by CVT and THM, respectively.},
keywords = {CuInS2, photoluminescence, exciton, Physics, Spectroscopy, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/19124/}
}

Eu was implanted into high quality cubic (zincblende) GaN (ZB-GaN) layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction (XRD) and Rutherford backscattering/channeling spectrometry. A low concentration ({\ensuremath{<}}10\%) of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their (0001) planes aligned with the \{111\} planes of the cubic lattice. Implantation of Eu causes an expansion of the lattice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN (W-GaN). For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the {\ensuremath{<}} 110 {\ensuremath{>}} direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence (PL), and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.

@Article{strathprints19085,
author = {K. Lorenz and I.S. Roqan and N. Franco and K.P. O'Donnell and V. Darakchieva and E. Alves and C. Trager-Cowan and R.W. Martin and D.J. As and M. Panfilova and Fundacao para a Ciencia e Tecnologia (FCT), Portugal (Funder) and HOYA Corporation (Funder) and German Science Foundation (DFG) (Funder) },
title = {Europium doping of zincblende GaN by ion implantation},
journal = {Journal of Applied Physics},
year = {2009},
volume = {105},
number = {11},
pages = {113507},
month = {June},
abstract = {Eu was implanted into high quality cubic (zincblende) GaN (ZB-GaN) layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction (XRD) and Rutherford backscattering/channeling spectrometry. A low concentration ({\ensuremath{<}}10\%) of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their (0001) planes aligned with the \{111\} planes of the cubic lattice. Implantation of Eu causes an expansion of the lattice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN (W-GaN). For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the {\ensuremath{<}} 110 {\ensuremath{>}} direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence (PL), and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.},
keywords = {EU-doped gan, cubic gan, optical activiation, lattice location, photoluminescence, growth, films, spectroscopy, epilayers, epitaxy, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/19085/}
}

GaN was implanted with 300 keV Eu ions over a wide fluence range from 1 {$\times$} 1013 to 1 {$\times$} 1016 Eu cm?2 at room temperature (RT) or 500 ?C. Detailed structural and optical characterizations of the samples were performed using Rutherford backscattering spectrometry and channelling, transmission and scanning electron microscopy, wavelength dispersive x-ray emission and RT cathodoluminescence (CL) spectroscopy. RT implantation results in a sigmoidal-shaped damage build-up curve with four regimes that were correlated with the formation of specific kinds of defects. After annealing at 1000 ?C only samples implanted to fluences below 0.8 {$\times$} 1015 Eu cm?2 showed near complete recovery of the crystal. Implantation at elevated temperature significantly decreases the implantation damage and increases the fraction of Eu incorporated on substitutional Ga-sites. The improved structural properties of samples implanted at elevated temperature are reflected in a higher intensity of Eu-related red light emission after annealing at 1000 ?C. The RT CL intensity is correlated with the number of Eu ions on substitutional Ga-sites after annealing. Furthermore, a detailed study of optical activation shows that the optimum annealing temperature depends on the implantation fluence due to the sensitive balance of defects removed and created during high temperature annealing.

Photoluminescence (PL) and PL excitation (PLE) spectra of Eu-implanted AlxGa1-xN are obtained across the whole alloy composition range. The dominant D-5(0)-F-7(2) emission band broadens and then narrows as x increases from 0 to 1 while the peak shifts monotonically. This behavior is surprisingly similar to the broadening of excitons in a semiconductor alloy caused by composition fluctuations [E. F. Schubert et al., Phys. Rev. B 30, 813 (1984). PLE spectra reveal a steplike AlxGa1-xN band-edge absorption and two "subgap" bands X-1,X-2:X-1 peaks at 3.26 eV in GaN and shifts linearly to 3.54 eV in AlN. For x {\ensuremath{>}} 0.6, X-2 emerges approximately 1 eV higher in energy than X-1 and shifts in a similar way. We propose that X-1,X-2 involve creation of core-excitonic complexes of Eu emitting centers.

Epitaxial layers of wurtzite-phase Al1?xInxN, 120 nm thick with (0 0 0 1) orientation, were grown by metal organic chemical vapour deposition on GaN buffer layers at setpoint temperatures between 760 and 840 ?C. For growth temperatures 800 ?C, the AlInN layers grew with uniform composition, pseudomorphic with the underlying GaN buffer layer. In the temperature range studied, the InN fractions are a linear function of the setpoint temperature and straddle the near-lattice-match composition around Al0.83In0.17N. Lowering the growth temperature to 760 ?C caused a compositional grading, a marked change in surface morphology, and a reduction in AlInN crystal quality. The resulting AlInN layer consists of a compressively strained interfacial layer with a composition of Al0.76In0.24N, and a mostly relaxed near-surface layer with a composition of Al0.81In0.19N. Atomic force microscopy suggests that a transition to a three-dimensional growth mode accompanies the structural relaxation and change in composition.

The behavior of rare earth dopants in GaN was investigated by means of theoretical techniques. The Density Functional based Tight-Binding method (DFTB) has been extended to include orbital dependent potentials (LDA+U and SIC-like) in attempt to model. the 4f states of lanthanide impurities within a realistic crystal model. We present results of an investigation into the structural and and energetic Properties of rare earth (Eu, Er and Tm) point defects in GaN. Lanthanide ions (either isolated or complexed with GaN native defects) prefer the Ga-site. Among the investigated defects the REGa V-N pairs are the most promising candidates as luminescent centers, while, interstitial lauthanides are found not to be compatible with the observed luminescence. Differences in the behavior of the single lanthanide ions are explained in term of different 4f-shell occupation and ion size.

@article{strathprints35316,
volume = {5},
number = {6},
month = {May},
author = {Simone Sanna and Benjamin Hourahine and Thomas Frauenheim and U. Gerstmann},
title = {Theoretical study of rare earth point defects in GaN},
journal = {Physica Status Solidi C},
pages = {2358--2360},
year = {2008},
keywords = {earth point defects , GaN , lanthanide impurities , crystals, interstitial lauthanides, Physics, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/35316/},
abstract = {The behavior of rare earth dopants in GaN was investigated by means of theoretical techniques. The Density Functional based Tight-Binding method (DFTB) has been extended to include orbital dependent potentials (LDA+U and SIC-like) in attempt to model. the 4f states of lanthanide impurities within a realistic crystal model. We present results of an investigation into the structural and and energetic Properties of rare earth (Eu, Er and Tm) point defects in GaN. Lanthanide ions (either isolated or complexed with GaN native defects) prefer the Ga-site. Among the investigated defects the REGa V-N pairs are the most promising candidates as luminescent centers, while, interstitial lauthanides are found not to be compatible with the observed luminescence. Differences in the behavior of the single lanthanide ions are explained in term of different 4f-shell occupation and ion size.}
}

The spectra of reflectance and luminescence of high-quality CuInS2 single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy {$\sim$}1.5356 eV and doubly degenerate peak BC at the energy {$\sim$}1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, A UPB at {$\sim$}1.5361 eV and A LPB at {$\sim$}1.5347 eV, with a half-width {$\sim$}1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be {$\sim$}18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed.

@Article{strathprints19828,
author = {A.V. Mudryi and A.V. Ivanyukovich and M.V. Yakushev and Robert Martin and A. Saad},
title = {Optical spectroscopy of free excitons in a CuInS₂ chalcopyrite semiconductor compound},
journal = {Semiconductors},
year = {2008},
volume = {42},
number = {1},
pages = {29--33},
month = {January},
abstract = {The spectra of reflectance and luminescence of high-quality CuInS2 single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy {$\sim$}1.5356 eV and doubly degenerate peak BC at the energy {$\sim$}1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, A UPB at {$\sim$}1.5361 eV and A LPB at {$\sim$}1.5347 eV, with a half-width {$\sim$}1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be {$\sim$}18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed.},
keywords = {cuInS2, optical spectroscopy, crystals, Physics, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/19828/}
}

The optical properties and fabrication of ultrathin GaN-based microcavities grown on silicon substrates are described. The epitaxial part of the optical cavities, consisting of a {\ensuremath{\lambda}}/2 GaN layer above a 3-period epitaxial Bragg mirror, is sandwiched between two silica/zirconia mirrors. At a suitable point in the fabrication process the silicon substrate was selectively removed using via holes. The cavity mode and excitonic resonance are observed by reflectivity at low and room temperature, demonstrating a quality factor of {\texttt{\char126}}125. The dispersion of the modes and their linewidth is measured using angle-resolved reflectivity and successfully modelled using transfer matrix simulations.

@Article{strathprints19734,
author = {K. Bejtka and P.R. Edwards and R.W. Martin and F. Reveret and A. Vasson and J. Leymarie and I.R. Sellers and M. Leroux},
title = {Fabrication and characterization of ultrathin double dielectric mirror GaN microcavities},
journal = {Semiconductor Science and Technology},
year = {2008},
volume = {23},
number = {4},
pages = {045008},
month = {April},
note = {Strathprints' policy is to record up to 8 authors per publication, plus any additional authors based at the University of Strathclyde. More authors may be listed on the official publication than appear in the Strathprints' record.},
abstract = {The optical properties and fabrication of ultrathin GaN-based microcavities grown on silicon substrates are described. The epitaxial part of the optical cavities, consisting of a {\ensuremath{\lambda}}/2 GaN layer above a 3-period epitaxial Bragg mirror, is sandwiched between two silica/zirconia mirrors. At a suitable point in the fabrication process the silicon substrate was selectively removed using via holes. The cavity mode and excitonic resonance are observed by reflectivity at low and room temperature, demonstrating a quality factor of {\texttt{\char126}}125. The dispersion of the modes and their linewidth is measured using angle-resolved reflectivity and successfully modelled using transfer matrix simulations.},
keywords = {silicon substrates, ultrathin GaN-based microcavities, optical properties, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/19734/}
}

This article introduces papers arising from the 14th Conference and Workshop on Electron Backscatter Diffraction(EBSD): its applications and related techniques,which was held between the 26th and 28th of March 2007 in New Lanark,Scotland.

Strong light-matter coupling is demonstrated at low temperature in an ultrathin GaN microcavity fabricated using two silica/zirconia Bragg mirrors, in addition to a three-period epitaxial (Al,Ga)N mirror serving as an etch stop and assuring good quality of the overgrown GaN. The {\ensuremath{\lambda}}/2 cavity is grown by molecular beam epitaxy on a Si substrate. Analysis of angle-resolved data reveal key features of the strong coupling regime in both reflectivity and transmission spectra at 5 K: anticrossing with a normal mode splitting of 43{$\pm$}2 meV and 56{$\pm$}2 meV for reflectivity and transmission, respectively, and narrowing of the lower polariton linewidth near resonance.

@Article{strathprints19621,
author = {K. Bejtka and F. Reveret and R.W. Martin and Paul Edwards and A. Vasson and J. Leymarie and I.R. Sellers and J.Y. Duboz},
title = {Strong light-matter coupling in ultrathin double dielectric mirror GaN microcavities},
journal = {Applied Physics Letters},
year = {2008},
volume = {92},
number = {24},
pages = {241105},
month = {June},
note = {Strathprints' policy is to record up to 8 authors per publication, plus any additional authors based at the University of Strathclyde. More authors may be listed on the official publication than appear in the Strathprints' record.},
abstract = {Strong light-matter coupling is demonstrated at low temperature in an ultrathin GaN microcavity fabricated using two silica/zirconia Bragg mirrors, in addition to a three-period epitaxial (Al,Ga)N mirror serving as an etch stop and assuring good quality of the overgrown GaN. The {\ensuremath{\lambda}}/2 cavity is grown by molecular beam epitaxy on a Si substrate. Analysis of angle-resolved data reveal key features of the strong coupling regime in both reflectivity and transmission spectra at 5 K: anticrossing with a normal mode splitting of 43{$\pm$}2 meV and 56{$\pm$}2 meV for reflectivity and transmission, respectively, and narrowing of the lower polariton linewidth near resonance.},
keywords = {semiconductor microcavities, photoluminescence, light-matter coupling, double dielectric mirror, GaN, Optics. Light, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/19621/}
}

We present a Raman scattering and cathodoluminescence study of a set of InxAl1-xN/GaN epilayers with InN fractions around the lattice-matched composition. We observed the A(1)(LO) and InN-like E-2 modes of the alloy, whose frequencies are in good agreement with theoretical predictions, but we were unable to detect the AlN-like E-2 mode. The InN-like E2 mode did not exhibit noticeable frequency shifts in the studied samples. This is explained by the presence of residual strain in the pseudomorphic InxAl1-xN films. A luminescence peak that shifts to lower energies with an increasing InN fraction was observed at energies above the band edge of the GaN substrate. The cathodoluminescence peak energy is lower than expected, indicating a large band-gap bowing in these alloy layers.

@Article{strathprints19490,
author = {R. Cusco and D. Pastor and S. Hernandez and L. Artus and O. Martinez and J. Jimenez and R.W. Martin and K.P. O'Donnell and I.M. Watson},
title = {Raman scattering and cathodoluminescence characterization of near lattice-matched InₓAl₁₋ₓN epilayers},
journal = {Semiconductor Science and Technology},
year = {2008},
volume = {23},
number = {10},
pages = {105002},
month = {October},
note = {Strathprints' policy is to record up to 8 authors per publication, plus any additional authors based at the University of Strathclyde. More authors may be listed on the official publication than appear in the Strathprints' record.},
abstract = {We present a Raman scattering and cathodoluminescence study of a set of InxAl1-xN/GaN epilayers with InN fractions around the lattice-matched composition. We observed the A(1)(LO) and InN-like E-2 modes of the alloy, whose frequencies are in good agreement with theoretical predictions, but we were unable to detect the AlN-like E-2 mode. The InN-like E2 mode did not exhibit noticeable frequency shifts in the studied samples. This is explained by the presence of residual strain in the pseudomorphic InxAl1-xN films. A luminescence peak that shifts to lower energies with an increasing InN fraction was observed at energies above the band edge of the GaN substrate. The cathodoluminescence peak energy is lower than expected, indicating a large band-gap bowing in these alloy layers.},
keywords = {alloys, films, Raman scattering, cathodoluminescence, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/19490/}
}

A study of AlInGaN epilayers, grown by plasma-assisted molecular beam epitaxy, was performed using spatially resolved x-ray microanalysis and luminescence spectroscopy in order to investigate competition between the incorporation of In, Al, and Ga as a function of the growth temperature in the 565-660 ?C range and the nominal AlN mole fraction. The samples studied have AlN and InN mole fractions in the ranges of 4\%-30\% and 0\%-16\%, respectively. Composition measurements show the effect of decreasing temperature to be an increase in the incorporation of InN, accompanied by a small but discernible decrease in the ratio of GaN to AlN mole fractions. The incorporation of In is also shown to be significantly increased by decreasing the Al mole fraction. Optical emission peaks, observed by cathodoluminescence mapping and by photoluminescence, provide further information on the epilayer compositions as a function of substrate temperature, and the dependencies of peak energy and linewidth are plotted.

@Article{strathprints19453,
author = {K. Bejtka and P.R. Edwards and R.W. Martin and S. Fernandez-Garrido and E. Calleja},
title = {Composition and luminescence of AlInGaN layers grown by plasma-assisted molecular beam epitaxy},
journal = {Journal of Applied Physics},
year = {2008},
volume = {104},
number = {7},
pages = {073537},
month = {October},
abstract = {A study of AlInGaN epilayers, grown by plasma-assisted molecular beam epitaxy, was performed using spatially resolved x-ray microanalysis and luminescence spectroscopy in order to investigate competition between the incorporation of In, Al, and Ga as a function of the growth temperature in the 565-660 ?C range and the nominal AlN mole fraction. The samples studied have AlN and InN mole fractions in the ranges of 4\%-30\% and 0\%-16\%, respectively. Composition measurements show the effect of decreasing temperature to be an increase in the incorporation of InN, accompanied by a small but discernible decrease in the ratio of GaN to AlN mole fractions. The incorporation of In is also shown to be significantly increased by decreasing the Al mole fraction. Optical emission peaks, observed by cathodoluminescence mapping and by photoluminescence, provide further information on the epilayer compositions as a function of substrate temperature, and the dependencies of peak energy and linewidth are plotted.},
keywords = {AlInGaN epilayers, plasma-assisted molecular beam epitaxy, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/19453/}
}

Oligonucleotide-gold-nanoparticle (OGN) conjugates are now recognized as both powerful tools for ultrasensitive detection and building blocks for the controlled creation of nanostructures (see image). The first use of both OGN and oligonucleotide-silver nanoparticle (OSN) conjugates are used to create mixed-metal nanostructures with novel optical properties. (Abstract from: http://www3.interscience.wiley.com/journal/121357719/abstract)

Near-lattice-matched GaN/Al1?xInxN single quantum wells, grown using both free-standing GaN and conventional GaN-on-sapphire substrates, are studied by photoluminescence (PL) and PL excitation spectroscopies. PL spectra distinguish luminescence originating in the wells, barriers, and underlying GaN buffer layers. The spectra also reveal significant differences between structures grown simultaneously on the different substrates. The quantum well transition energy decreases as the well width increases due to the intense in-built electric fields, estimated to be 3.0{$\pm$}0.5 MeV/cm, that persist in strain free GaN/Al1?xInxN. Screening of these fields is studied using the excitation power dependence of the PL

An array of GaN micropyramids containing a near-surface InxGa1-xN/GaN single quantum well has been fabricated using selective area epitaxial overgrowth above a patterned silica mask. The pyramid array has been studied by means of angle-resolved reflection measurements using s- and p-polarized incident light in the near- and mid-infrared optical ranges. We have found that the periodic array of flat-topped pyramids shows marked resonances in the near-infrared optical range due to resonant Bloch modes within the extraction cone and that the angular dispersion of these modes exhibits strong photonic crystal characteristics. The experimental results are in good agreement with the photonic band structure calculated using a scattering matrix formalism. The mid-infrared optical anisotropy properties of the micropyramids were investigated to probe the infrared active phonons of the pyramid array. The A1(LO) phonon of the InxGa1-xN/GaN single quantum well was identified and the InN mole fraction was estimated from the mode behavior

@Article{strathprints14013,
author = {D. Coquillat and M. Le Vassor D'Yerville and M. Kazan and C. Liu and I.M. Watson and P.R. Edwards and R.W. Martin and H.M.H. Chong and R.M. De La Rue},
title = {Studies of the photonic and optical-frequency phonon properties of selectively grown GaN micro-pyramids},
journal = {Journal of Applied Physics},
year = {2008},
volume = {103},
number = {4},
pages = {004910},
month = {February},
abstract = {An array of GaN micropyramids containing a near-surface InxGa1-xN/GaN single quantum well has been fabricated using selective area epitaxial overgrowth above a patterned silica mask. The pyramid array has been studied by means of angle-resolved reflection measurements using s- and p-polarized incident light in the near- and mid-infrared optical ranges. We have found that the periodic array of flat-topped pyramids shows marked resonances in the near-infrared optical range due to resonant Bloch modes within the extraction cone and that the angular dispersion of these modes exhibits strong photonic crystal characteristics. The experimental results are in good agreement with the photonic band structure calculated using a scattering matrix formalism. The mid-infrared optical anisotropy properties of the micropyramids were investigated to probe the infrared active phonons of the pyramid array. The A1(LO) phonon of the InxGa1-xN/GaN single quantum well was identified and the InN mole fraction was estimated from the mode behavior},
keywords = {epitaxial growth, gallium compounds, III-V semiconductors, indium compounds, phonons, photonic crystals, semiconductor epitaxial layers, semiconductor quantum wells, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/14013/}
}

Processing of GaN-AlInN-GaN epitaxial trilayers into 3-dimensional microstructures, using a combination of vertical dry etching and lateral wet etching, is discussed. The AlInN layers were grown so as to have an InN mole fraction close to the value of 17\% required for lattice matching with GaN. Inductively coupled plasma etching with chlorine-argon gas mixtures was used to define mesa features with near-vertical sidewalls. Refluxing aqueous solutions of nitric acid of 2 molar concentration allowed highly selective lateral etching of the AlInN interlayers exposed on the mesa sidewalls, providing a novel sacrificial layer technology for the III-nitride materials. Lateral etch rates of 0.14-0.21 {\ensuremath{\mu}}m/hr were observed for 100-nm AlInN interlayers. Two distinct applications are discussed. In one example, lateral etching of an AlInN layer was used to expose the underside of epitaxial GaN disks for fabrication of planar microcavities. Here, retention of an optically smooth GaN (0001) surface on the underside of the disks is critical. Microbridges with potential for development as sensors were also demonstrated, and the deformation of these structures provides a sensitive probe of the local strain state of the undercut GaN layer.

@Article{strathprints14006,
author = {I.M. Watson and C. Xiong and E. Gu and M.D. Dawson and F. Rizzi and K. Bejtka and P.R. Edwards and R.W. Martin},
title = {Selective wet etching of AlInN layers for nitride-based MEMS and photonic device structures},
journal = {Proceedings of SPIE the International Society for Optical Engineering},
year = {2008},
volume = {6993},
pages = {69930E},
month = {April},
note = {Available in: MEMS, MOEMS, and Micromachining III (Proceedings Volume) Proceedings of SPIE Volume: 6993 Editor(s): Hakan Urey Date: 28 May 2008 ISBN: 9780819471918},
abstract = {Processing of GaN-AlInN-GaN epitaxial trilayers into 3-dimensional microstructures, using a combination of vertical dry etching and lateral wet etching, is discussed. The AlInN layers were grown so as to have an InN mole fraction close to the value of 17\% required for lattice matching with GaN. Inductively coupled plasma etching with chlorine-argon gas mixtures was used to define mesa features with near-vertical sidewalls. Refluxing aqueous solutions of nitric acid of 2 molar concentration allowed highly selective lateral etching of the AlInN interlayers exposed on the mesa sidewalls, providing a novel sacrificial layer technology for the III-nitride materials. Lateral etch rates of 0.14-0.21 {\ensuremath{\mu}}m/hr were observed for 100-nm AlInN interlayers. Two distinct applications are discussed. In one example, lateral etching of an AlInN layer was used to expose the underside of epitaxial GaN disks for fabrication of planar microcavities. Here, retention of an optically smooth GaN (0001) surface on the underside of the disks is critical. Microbridges with potential for development as sensors were also demonstrated, and the deformation of these structures provides a sensitive probe of the local strain state of the undercut GaN layer.},
keywords = {photonics, GaN layers, vertical dry etching, lateral wet etching, Optics. Light, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Applied Mathematics, Computer Science Applications},
url = {http://strathprints.strath.ac.uk/14006/}
}

Optical energy gaps are measured for high-quality Al1?xInxN-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of 6 eV and differences with earlier reports are discussed. Very large Stokes’ shifts of 0.4-0.8 eV are observed in the composition range 0.13{\ensuremath{<}}x{\ensuremath{<}}0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field

@Article{strathprints14005,
author = {K. Wang and R. W. Martin and D. Amabile and P. R. Edwards and S. Hernandez and E. Nogales and K. P. O'Donnell and K. Lorenz and E. Alves and V. Matias and A. Vantomme and D. Wolverson and I. M. Watson},
title = {Optical energies of AllnN epilayers},
journal = {Journal of Applied Physics},
year = {2008},
volume = {103},
number = {7},
pages = {073510},
month = {April},
abstract = {Optical energy gaps are measured for high-quality Al1?xInxN-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of 6 eV and differences with earlier reports are discussed. Very large Stokes' shifts of 0.4-0.8 eV are observed in the composition range 0.13{\ensuremath{<}}x{\ensuremath{<}}0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field},
keywords = {aluminium compounds, indium compounds, photoluminescence, semiconductor epitaxial layers, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/14005/}
}

Cathodoluminescence (CL) and Photoluminescence (PL) of Eu-implanted zincblende-GaN (ZB-GaN:Eu) and wurtzite-GaN (W-GaN:Eu) are compared in order to investigate the optical activation of GaN by Eu. The EU3+ emission spectrum depends critically on the crystal structure of the GaN host; implantation and post-annealing at 800 degrees C partially converts implantation-damaged ZB-GaN:Eu to W-GaN:Eu. Selective excitation of PL at wavelengths below the ZB-GaN band edge reveals a new sharp emission line at 627 nm, together with a number of satellites, which we ascribe to ZB-GaN: Eu.

The implantation damage and rare earth (RE) luminescence in the wide band gap ternaries AlGaN and AlInN were studied and compared to GaN. For both ternaries lower damage levels were observed and in contrast to GaN, no surface amorphisation occurs during the implantation. Damage recovery of RE implanted GaN was studied for post implant annealing at temperatures between 800 ?C and 1300 ?C. The blue and IR Tm related luminescence intensity is seen to increase strongly with the annealing temperature. The two Tm lines observed at 478 nm and 465 nm are assigned to the 1G4 3H6 and the 1D23F4 transition, respectively. For GaN the line at 465 nm is fully quenched at RT while it becomes the dominant line for the ternaries. The blue luminescence intensity in the ternaries is significantly stronger than in GaN. Furthermore, AlInN shows a very high ratio of blue/IR luminescence.

@article{strathprints13951,
volume = {205},
number = {1},
month = {January},
author = {K. Lorenz and E. Alves and I.S. Roqan and R.W. Martin and C. Trager-Cowan and K.P. O'Donnell and I.M. Watson},
title = {Rare earth doping of III-nitride alloys by ion implantation},
journal = {Physica Status Solidi A},
pages = {34--37},
year = {2008},
keywords = {rare earth luminescence, wide band gap, AlGaN, AlInN, GaN, Optics. Light, Materials Chemistry, Surfaces, Coatings and Films, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/13951/},
abstract = {The implantation damage and rare earth (RE) luminescence in the wide band gap ternaries AlGaN and AlInN were studied and compared to GaN. For both ternaries lower damage levels were observed and in contrast to GaN, no surface amorphisation occurs during the implantation. Damage recovery of RE implanted GaN was studied for post implant annealing at temperatures between 800 ?C and 1300 ?C. The blue and IR Tm related luminescence intensity is seen to increase strongly with the annealing temperature. The two Tm lines observed at 478 nm and 465 nm are assigned to the 1G4 3H6 and the 1D23F4 transition, respectively. For GaN the line at 465 nm is fully quenched at RT while it becomes the dominant line for the ternaries. The blue luminescence intensity in the ternaries is significantly stronger than in GaN. Furthermore, AlInN shows a very high ratio of blue/IR luminescence.}
}

This paper describes processing of GaN on the on the (000I) N-face surface, using two different high-density plasma etch techniques, inductively coupled plasma (ICP) etch, and electron cyclotron resonance (ECR) etching. ICP experiments used several different conditions employing Cl2-Ar-BCl3 or Cl2-Ar plasmas. The resulting maximum etch rates of 370-390 nm/min are approximately twice as high as etch rates for Ga-face (0001) GaN with the same recipes. ECR etching employed a Cl2-CH4-Ar recipe, which produced an average etch rate of 55 nm/min in a 20-minute etch process on N-face GaN. Both etch techniques increased the roughness of N-face GaN, but could produce surfaces with average roughness values below 3 nm. Selection of conditions with a dominant chemical etch contribution is important to maintain smooth surfaces. The use of both ICP and ECR etching in sequence is advantageous in situations where a GaN substrate several tens of microns in thickness must be thinned from the backside, stopping the etch in a suitable marker layer.

@article{strathprints9111,
volume = {4},
number = {1},
title = {Dry etching of n-face GaN using two high-density plasma etch techniques},
author = {F. Rizzi and K. Bejtka and F. Semond and E. Gu and M.D. Dawson and I.M. Watson and R.W. Martin},
year = {2007},
pages = {200--2003},
journal = {Physica Status Solidi C},
keywords = {high-density plasma, dry etching, etch techniques, inductively coupled plasma, electron cyclotron resonance, Optics. Light, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/9111/},
abstract = {This paper describes processing of GaN on the on the (000I) N-face surface, using two different high-density plasma etch techniques, inductively coupled plasma (ICP) etch, and electron cyclotron resonance (ECR) etching. ICP experiments used several different conditions employing Cl2-Ar-BCl3 or Cl2-Ar plasmas. The resulting maximum etch rates of 370-390 nm/min are approximately twice as high as etch rates for Ga-face (0001) GaN with the same recipes. ECR etching employed a Cl2-CH4-Ar recipe, which produced an average etch rate of 55 nm/min in a 20-minute etch process on N-face GaN. Both etch techniques increased the roughness of N-face GaN, but could produce surfaces with average roughness values below 3 nm. Selection of conditions with a dominant chemical etch contribution is important to maintain smooth surfaces. The use of both ICP and ECR etching in sequence is advantageous in situations where a GaN substrate several tens of microns in thickness must be thinned from the backside, stopping the etch in a suitable marker layer.}
}

Selective area growth offers the promise of producing III-nitride photonic crystals (PhCs) without dry etching and concomitant surface damage. Two PhC structures were studied, one comprising an array of flat-topped GaN micropyramids with micrometre-scale periodicity, and the other made up of sharp-tipped pyramids with sub-micrometre periodicity. Angularly resolved reflection and transmission measurements revealed the presence of sharp resonances associated with resonant Bloch modes. As a result, the photonic band structure was determined along symmetry directions of the reciprocal lattice for the two PhC structures.

Wet etching of AlInN-GaN epitaxial heterostructures, containing AlInN layers with InN mole fractions close to 0.17 has been studied. One molar aqueous solution of the chelating amine 1,2-diaminoethane (DAE) proved to selectively etch the AlInN layers, without the need for heating above room temperature, or photo-assistance. In experiments with a (0 0 0 1)-oriented AlInN-on-GaN bilayer, the mode of removal of the AlInN layer was predominantly lateral etching, initiated from the sidewalls of pit defects in the AlInN layer. The lateral etch rate was estimated at 60 nm/h. The GaN buffer layer surface was roughened concurrently with etching of the AlInN, although the DAE solution has no effect on as-grown GaN (0 0 0 1) surfaces. The roughening of the GaN surface is tentatively attributed to the charge accumulation layer expected at the AlInN-GaN heterointerface. The DAE etchant also proved effective at removing buried AlInN layers from trilayer and more complex multilayer structures, leading to the prospect of epitaxial lift-off processes, and the fabrication of three-dimensional engineered microstructures. These capabilities were demonstrated by the production of suspended microdisk structures from a GaN-AlInN-GaN trilayer, using a combination of dry and wet etching.

@article{strathprints9069,
volume = {300},
number = {1},
month = {March},
author = {F. Rizzi and K. Bejtka and P.R. Edwards and R.W. Martin and I.M. Watson},
title = {Selective wet etching of lattice-matched AlInN-GaN heterostructures},
journal = {Journal of Crystal Growth},
pages = {254--258},
year = {2007},
keywords = {etching, metalorganic vapor phase epitaxy, nitrides, semiconducting III?V materials, Optics. Light, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/9069/},
abstract = {Wet etching of AlInN-GaN epitaxial heterostructures, containing AlInN layers with InN mole fractions close to 0.17 has been studied. One molar aqueous solution of the chelating amine 1,2-diaminoethane (DAE) proved to selectively etch the AlInN layers, without the need for heating above room temperature, or photo-assistance. In experiments with a (0 0 0 1)-oriented AlInN-on-GaN bilayer, the mode of removal of the AlInN layer was predominantly lateral etching, initiated from the sidewalls of pit defects in the AlInN layer. The lateral etch rate was estimated at 60 nm/h. The GaN buffer layer surface was roughened concurrently with etching of the AlInN, although the DAE solution has no effect on as-grown GaN (0 0 0 1) surfaces. The roughening of the GaN surface is tentatively attributed to the charge accumulation layer expected at the AlInN-GaN heterointerface. The DAE etchant also proved effective at removing buried AlInN layers from trilayer and more complex multilayer structures, leading to the prospect of epitaxial lift-off processes, and the fabrication of three-dimensional engineered microstructures. These capabilities were demonstrated by the production of suspended microdisk structures from a GaN-AlInN-GaN trilayer, using a combination of dry and wet etching.}
}

Comparable microcavities with 3/2 ({\texttt{\char126}}240 nm) active regions containing distributed (In,Ga)N quantum wells, grown on GaN substrates and bounded by two dielectric mirrors, have been fabricated by two different routes: one using laser lift-off to process structures grown on GaN-on-sapphire templates and the second using freestanding GaN substrates, which are initially processed by mechanical thinning. Both exploit the properties of an Al0.83In0.17N layer, lattice matched to the GaN substrate and spacer layers. In both cases cavity quality factors {\ensuremath{>}}400 are demonstrated by measurements of the cavity-filtered room-temperature excitonic emission near 410 nm.

We describe the fabrication and optical properties of a 3{\ensuremath{\lambda}}/2 InGaN/GaN-based microcavity using "upper" and "lower" silica/zirconia mirrors. The fabrication of this structure involved selective removal of an AlInN layer following multistep thinning of a free-standing GaN substrate. Photoluminescence spectra show a narrowing of the excitonic emission from InGaN/GaN quantum wells in the microcavity, giving a cavity quality factor Q exceeding 400.

The processing of N-polar GaN (000 (1) over bar) samples has been studied, motivated by applications in which extensive back side thinning of freestanding GaN (FS-GaN) substrates is required. Experiments were conducted on FS-GaN from two commercial sources, in addition to epitaxial GaN with the N-face exposed by a laser lift-off process. The different types of samples produced equivalent results. Surface morphologies were examined over relatively large areas, using scanning electron microscopy and stylus profiling. The main focus of this study was on inductively coupled plasma (ICP) etch processes, employing Cl-2/Ar or Cl-2/BCl3Ar gas mixtures. Application of a standard etch recipe, optimized for feature etching of Ga-polar GaN (0001) surfaces, caused severe roughening of N-polar samples and confirmed the necessity for specific optimization of etch conditions for N-face material. A series of recipes with a reduced physical (sputter-based) contribution to etching allowed average surface roughness values to be consistently reduced to below 3 nm. Maximum N-face etch rates of 370-390 nm/min have been obtained in recipes examined to date. These are typically faster than etch rates obtained on Ga-face samples under the same conditions and adequate for the process flows of interest. Mechanistic aspects of the ICP etch process and possible factors contributing to residual surface roughness are discussed. This study also included work on chemomechanical polishing (CMP). The optimized CMP process had stock removal rates of similar to 500 nm/h on the GaN N face. This was much slower than the ICP etching but showed the important capability of recovering smooth surfaces on samples roughened in previous processing. In one example, a surface roughened by nonoptimized ICP etching was smoothed to give an average surface roughness of similar to 2 nm.

@article{strathprints5317,
volume = {25},
number = {2},
title = {Thinning of N-face GaN (000-1) samples by inductively coupled plasma etching and chemomechanical polishing},
author = {F. Rizzi and E. Gu and M.D. Dawson and I.M. Watson and R.W. Martin and X.N. Kang and G.Y. Zhang},
year = {2007},
pages = {252--260},
journal = {Journal of Vacuum Science and Technology A},
keywords = {semiconductors, fabrication, etching, plasmas, photonics, optics, Optics. Light, Surfaces, Coatings and Films, Surfaces and Interfaces, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/5317/},
abstract = {The processing of N-polar GaN (000 (1) over bar) samples has been studied, motivated by applications in which extensive back side thinning of freestanding GaN (FS-GaN) substrates is required. Experiments were conducted on FS-GaN from two commercial sources, in addition to epitaxial GaN with the N-face exposed by a laser lift-off process. The different types of samples produced equivalent results. Surface morphologies were examined over relatively large areas, using scanning electron microscopy and stylus profiling. The main focus of this study was on inductively coupled plasma (ICP) etch processes, employing Cl-2/Ar or Cl-2/BCl3Ar gas mixtures. Application of a standard etch recipe, optimized for feature etching of Ga-polar GaN (0001) surfaces, caused severe roughening of N-polar samples and confirmed the necessity for specific optimization of etch conditions for N-face material. A series of recipes with a reduced physical (sputter-based) contribution to etching allowed average surface roughness values to be consistently reduced to below 3 nm. Maximum N-face etch rates of 370-390 nm/min have been obtained in recipes examined to date. These are typically faster than etch rates obtained on Ga-face samples under the same conditions and adequate for the process flows of interest. Mechanistic aspects of the ICP etch process and possible factors contributing to residual surface roughness are discussed. This study also included work on chemomechanical polishing (CMP). The optimized CMP process had stock removal rates of similar to 500 nm/h on the GaN N face. This was much slower than the ICP etching but showed the important capability of recovering smooth surfaces on samples roughened in previous processing. In one example, a surface roughened by nonoptimized ICP etching was smoothed to give an average surface roughness of similar to 2 nm.}
}

The density functional based tight-binding (DFTB) method can benefit substantially from a number of developments in density functional theory (DFT) while also providing a simple analytical proving ground for new extensions. This contribution begins by demonstrating the variational nature of charge-self-consistent DFTB (SCC-DFTB), proving the presence of a defined ground-state in this class of methods. Because the ground state of the SCC-DFTB method itself can be qualitatively incorrect for some systems, suitable forms of the recent LDA+U functionals for SCC-DFTB are also presented. This leads to both a new semilocal self-interaction correction scheme and a new physical argument for the choice of parameters in the LDA+U method. The locality of these corrections can only partly repair the HOMO-LUMO gap and chemical potential discontinuity, hence a novel method for introducing this further physics into the method is also presented, leading to exact derivative discontinuities in this theory at low computational cost. The prototypical system NiO is used as an illustration for these developments.

@Article{strathprints5105,
author = {B. Hourahine and S. Sanna and B. Aradi and C. Kohler and T. Niehaus and T. Frauenheim},
title = {Self-interaction and strong correlation in DFTB},
journal = {Journal of Physical Chemistry A},
year = {2007},
volume = {111},
number = {26},
pages = {5671--5677},
month = {July},
abstract = {The density functional based tight-binding (DFTB) method can benefit substantially from a number of developments in density functional theory (DFT) while also providing a simple analytical proving ground for new extensions. This contribution begins by demonstrating the variational nature of charge-self-consistent DFTB (SCC-DFTB), proving the presence of a defined ground-state in this class of methods. Because the ground state of the SCC-DFTB method itself can be qualitatively incorrect for some systems, suitable forms of the recent LDA+U functionals for SCC-DFTB are also presented. This leads to both a new semilocal self-interaction correction scheme and a new physical argument for the choice of parameters in the LDA+U method. The locality of these corrections can only partly repair the HOMO-LUMO gap and chemical potential discontinuity, hence a novel method for introducing this further physics into the method is also presented, leading to exact derivative discontinuities in this theory at low computational cost. The prototypical system NiO is used as an illustration for these developments.},
keywords = {density, tight-binding, DFTB, nanoscience, applied physics, Solid state physics. Nanoscience, Physical and Theoretical Chemistry},
url = {http://strathprints.strath.ac.uk/5105/}
}

The most time-consuming part of developing new parametrizations for the density functional based tight-binding (DFTB) method consists of producing accurate and transferable repulsive pair potentials. In the conventional approach to repulsive parametrization, every possible diatomic combination of the elements covered by the parametrization must be individually hand-constructed. We present an initial attempt to automate some of this time-consuming process. We consider a simple genetic algorithm-based approach to the fitting problem.

We report benchmark calculations of the density functional based tight-binding method concerning the magnetic properties of small iron clusters (Fe-2 to Fe-5) and the Fe-13 icosahedron. Energetics and stability with respect to changes of cluster geometry of collinear and noncollinear spin configurations are in good agreement with ab initio results. The inclusion of spin-orbit coupling has been tested for the iron dimer.

The functionals usually applied in DFT calculations have deficiencies in describing systems with strongly localized electrons such as transition metals or rare earth (RE) compounds. In this work, we present the self-consistent charge density based functional tight binding (SCC-DFTB) calculation scheme including LDA+U like potentials and apply it for the simulation of RE-doped GaN. DFTB parameters for the simulation of GaN and a selection of rare earth ions, where the f electrons were explicitly included in the valence, have been created. The results of the simulations were tested against experimental data (where present) and against various more sophisticated but computationally more costly DFT calculations. Our approach is found to correctly reproduce the geometry and the energetic of the studied systems.

@article{strathprints31167,
volume = {111},
number = {26},
month = {July},
author = {Simone Sanna and B. Hourahine and Th. Gallauner and Th. Frauenheim},
title = {An efficient LDA+U based tight binding approach},
journal = {Journal of Physical Chemistry A},
pages = {5665--5670},
year = {2007},
keywords = {self interaction correction, density functional theory, absorption fine structure, doped GaN, electronic structure, complex materials, lattice location, Gallium Nitride , implanted GaN, erbium, Physics, Physical and Theoretical Chemistry},
url = {http://strathprints.strath.ac.uk/31167/},
abstract = {The functionals usually applied in DFT calculations have deficiencies in describing systems with strongly localized electrons such as transition metals or rare earth (RE) compounds. In this work, we present the self-consistent charge density based functional tight binding (SCC-DFTB) calculation scheme including LDA+U like potentials and apply it for the simulation of RE-doped GaN. DFTB parameters for the simulation of GaN and a selection of rare earth ions, where the f electrons were explicitly included in the valence, have been created. The results of the simulations were tested against experimental data (where present) and against various more sophisticated but computationally more costly DFT calculations. Our approach is found to correctly reproduce the geometry and the energetic of the studied systems.}
}

A new Fortran 95 implementation of the DFTB (density functional-based tight binding) method has been developed, where the sparsity of the DFTB system of equations has been exploited. Conventional dense algebra is used only to evaluate the eigenproblems of the system and long-range Coulombic terms, but drop-in O(N) or O(N-2) modules are planned to replace the small code sections that these entail. The developed sparse storage structure is discussed in detail, and a short overview of other features of the new code is given.

@article{strathprints31166,
volume = {111},
number = {26},
month = {July},
author = {B. Aradi and B. Hourahine and Th. Frauenheim},
title = {DFTB+, a sparse matrix-based implementation of the DFTB method},
journal = {Journal of Physical Chemistry A},
pages = {5678--5684},
year = {2007},
keywords = {electronic structur calculations, density functional theory, tight binding method, ewald sums, convergence, simulations, potentials, molecules, Physics, Physical and Theoretical Chemistry},
url = {http://strathprints.strath.ac.uk/31166/},
abstract = {A new Fortran 95 implementation of the DFTB (density functional-based tight binding) method has been developed, where the sparsity of the DFTB system of equations has been exploited. Conventional dense algebra is used only to evaluate the eigenproblems of the system and long-range Coulombic terms, but drop-in O(N) or O(N-2) modules are planned to replace the small code sections that these entail. The developed sparse storage structure is discussed in detail, and a short overview of other features of the new code is given.}
}

In this work, we present results from self-consistent charge density functional based tight-binding (DFTB) calculational scheme, including local-density approximation +U (LDA+U) and simplified self-interaction-corrected-like potentials for the simulation of systems with localized strongly correlated electrons. This approach attempts to combine the efficiency of tight binding with the accuracy of more sophisticated ab initio methods and allows treatment of highly correlated electrons for very large systems. This is particularly interesting for the case of rare earths in GaN, where dilute amount of rare earth ions is used. In this work, we show the results of test calculations on bulk ErN and on the substitutional Er-Ga in wurtzite GaN, which we choose as representatives of bulk and point defects in solids with strongly correlated electrons. We find that ErN is a half metal in the ferromagnetic phase and that the substitutional Er-Ga in wurtzite GaN has C-3v symmetry. These examples show that the DFTB approach reproduces well the results of more demanding calculation schemes with a very low computational cost, making it suitable for the study of extended systems beyond the capabilities of density functional theory.

@Article{strathprints31163,
author = {Simone Sanna and B. Hourahine and U. Gerstmann and Th. Frauenheim},
title = {Efficient tight-binding approach for the study of strongly correlated systems},
journal = {Physical Review B},
year = {2007},
volume = {76},
number = {15},
pages = {155128},
month = {October},
abstract = {In this work, we present results from self-consistent charge density functional based tight-binding (DFTB) calculational scheme, including local-density approximation +U (LDA+U) and simplified self-interaction-corrected-like potentials for the simulation of systems with localized strongly correlated electrons. This approach attempts to combine the efficiency of tight binding with the accuracy of more sophisticated ab initio methods and allows treatment of highly correlated electrons for very large systems. This is particularly interesting for the case of rare earths in GaN, where dilute amount of rare earth ions is used. In this work, we show the results of test calculations on bulk ErN and on the substitutional Er-Ga in wurtzite GaN, which we choose as representatives of bulk and point defects in solids with strongly correlated electrons. We find that ErN is a half metal in the ferromagnetic phase and that the substitutional Er-Ga in wurtzite GaN has C-3v symmetry. These examples show that the DFTB approach reproduces well the results of more demanding calculation schemes with a very low computational cost, making it suitable for the study of extended systems beyond the capabilities of density functional theory.},
keywords = {density functional theory, electronic structure, self interaction, complex materials, Gallium Nitride, III-Nitrides, GaN, erbium, ER, simulations, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31163/}
}

InGaN is the basis of a new generation of light-emitting devices, with enormous technological potential; it is currently one of the most intensively studied semiconductor materials. It is generally accepted that compositional fluctuations resulting from phase segregation are the origin of the high luminescence efficiency of InGaN. Evidence to show that nanoscale strain inhomogeneity plays a fundamental role in determining the spectral properties of InGaN-GaN heterostrucures is reported. For layers above a certain critical thickness, a strong spatially varying strain profile accompanies a nonplanar surface morphology, which is associated with a transition from a planar 2D to a Stranski-Krastanow-like-2D-3D growth mode; the strong dependence of the critical thickness on the local InN content of the growing films drives a non-linear growth instability. Within this framework, apparently disparate experimental observations regarding structural and optical properties,previously reported for InGaN layers, are reconciled by a simple phenomenological description.

@Article{strathprints31162,
author = {de Sousa Pereira, Sergio Manuel and Kevin Peter O'Donnell and Eduardo Jorge da Costa Alves},
title = {Role of nanoscale strain inhomogeneity on the light emission from InGaN epilayers},
journal = {Advanced Functional Materials},
year = {2007},
volume = {17},
number = {1},
pages = {37--42},
month = {January},
abstract = {InGaN is the basis of a new generation of light-emitting devices, with enormous technological potential; it is currently one of the most intensively studied semiconductor materials. It is generally accepted that compositional fluctuations resulting from phase segregation are the origin of the high luminescence efficiency of InGaN. Evidence to show that nanoscale strain inhomogeneity plays a fundamental role in determining the spectral properties of InGaN-GaN heterostrucures is reported. For layers above a certain critical thickness, a strong spatially varying strain profile accompanies a nonplanar surface morphology, which is associated with a transition from a planar 2D to a Stranski-Krastanow-like-2D-3D growth mode; the strong dependence of the critical thickness on the local InN content of the growing films drives a non-linear growth instability. Within this framework, apparently disparate experimental observations regarding structural and optical properties,previously reported for InGaN layers, are reconciled by a simple phenomenological description.},
keywords = {critical layer thickness , nanocluster induced luminescence, multiple quantum wells, low stokes shift, band-gap, heterostructures, dependence, epifilms, nitride, origin, Physics, Biomaterials, Electrochemistry, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31162/}
}

Energy scalability of the excitation-emission spectra of InGaN epilayers, quantum wells and light-emitting diodes provided indirect evidence for a fundamental common cause of the remarkable optical properties of this commercially important semiconductor alloy. Phase segregation on the nanoscale ( accidental quantum dot formation) has generally been accepted as the mechanism of the spectral energy scaling ( K. P. O’Donnell, R. W. Martin and P. G. Middleton, Phys. Rev. Lett. 82 237 ( 1999)). Recently, however, the downsizing of the InN bandgap, from 2 to about 1 eV, has prompted a re-examination of the observations. Here, we present new structural evidence of InGaN nanostructure, obtained from a comparative analysis of Ga and In K-edge EXAFS ( extended X-ray absorption fine structure) of a wide range of InxGa1-xN epilayer samples. The mean In-Ga and Ga-In next-nearest-neighbour ( NNN) separations are found to be unequal in length for InN-poor ( 0.1 {\ensuremath{<}} x {\ensuremath{<}} 0.4) samples. The degree of inequality increases with decreasing InN fraction, x, and therefore correlates with luminescence efficiency in this range of alloy composition. We propose that the breakdown of In/Ga randomicity in InGaN alloys is associated with efficient excitation-emission in blue-green light-emitting devices. Although non-randomicity may lead to a weak quasi-localization of excitation, through the suppression of energy back-transfer, the issue of strong exciton localization in InGaN is not directly addressed by these results.

@article{strathprints31044,
volume = {87},
number = {13},
title = {Localization of excitation in InGaN epilayers},
author = {V. Kachkanov and K. P. O'Donnell and S. Pereira and R. W. Martin},
year = {2007},
pages = {1999--2017},
journal = {Philosophical Magazine},
keywords = {luminescence, excitation, emissions, absorption, Optics. Light},
url = {http://strathprints.strath.ac.uk/31044/},
abstract = {Energy scalability of the excitation-emission spectra of InGaN epilayers, quantum wells and light-emitting diodes provided indirect evidence for a fundamental common cause of the remarkable optical properties of this commercially important semiconductor alloy. Phase segregation on the nanoscale ( accidental quantum dot formation) has generally been accepted as the mechanism of the spectral energy scaling ( K. P. O'Donnell, R. W. Martin and P. G. Middleton, Phys. Rev. Lett. 82 237 ( 1999)). Recently, however, the downsizing of the InN bandgap, from 2 to about 1 eV, has prompted a re-examination of the observations. Here, we present new structural evidence of InGaN nanostructure, obtained from a comparative analysis of Ga and In K-edge EXAFS ( extended X-ray absorption fine structure) of a wide range of InxGa1-xN epilayer samples. The mean In-Ga and Ga-In next-nearest-neighbour ( NNN) separations are found to be unequal in length for InN-poor ( 0.1 {\ensuremath{<}} x {\ensuremath{<}} 0.4) samples. The degree of inequality increases with decreasing InN fraction, x, and therefore correlates with luminescence efficiency in this range of alloy composition. We propose that the breakdown of In/Ga randomicity in InGaN alloys is associated with efficient excitation-emission in blue-green light-emitting devices. Although non-randomicity may lead to a weak quasi-localization of excitation, through the suppression of energy back-transfer, the issue of strong exciton localization in InGaN is not directly addressed by these results.}
}

In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging-in the scanning electron microscope-to study tilt, atomic steps and dislocations in epitaxial GaN thin films. We show results from a series of GaN thin films of increasing thickness and from a just coalesced epitaxial laterally overgrown GaN thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02 degrees, in GaN thin films. As EBSD has a spatial resolution of approximate to 20 nm, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that electron channeling contrast images may be used to image tilt, atomic steps, and threading dislocations in GaN thin films.

@Article{strathprints31032,
author = {C. Trager-Cowan and F. Sweeney and P. W. Trimby and A. P. Day and A. Gholinia and N. -H. Schmidt and P. J. Parbrook and A. J. Wilkinson and I. M. Watson},
title = {Electron backscatter diffraction and electron channeling contrast imaging of tilt and dislocations in nitride thin films},
journal = {Physical Review B},
year = {2007},
volume = {75},
number = {8},
pages = {085301},
month = {February},
abstract = {In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging-in the scanning electron microscope-to study tilt, atomic steps and dislocations in epitaxial GaN thin films. We show results from a series of GaN thin films of increasing thickness and from a just coalesced epitaxial laterally overgrown GaN thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02 degrees, in GaN thin films. As EBSD has a spatial resolution of approximate to 20 nm, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that electron channeling contrast images may be used to image tilt, atomic steps, and threading dislocations in GaN thin films.},
keywords = {plan-view image, kikuchi diffraction, microscope, rocks, Plasma physics. Ionized gases, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31032/}
}

We present an approach for the simulation of complete electron backscatter diffraction (EBSD) patterns where the relative intensity distributions in the patterns are accurately reproduced. The Bloch wave theory is applied to describe the electron diffraction process. For the simulation of experimental patterns with a large field of view, a large number of reflecting planes has to be taken into account. This is made possible by the Bethe perturbation of weak reflections. Very good agreement is obtained for simulated and experimental patterns of gallium nitride GaNf0001g at 20 kV electron energy. Experimental features like zone-axis fine structure and higher-order Laue zone rings are accurately reproduced. We discuss the influence of the diffraction of the incident beam in our experiment.

@article{strathprints3089,
volume = {107},
number = {2007},
title = {Many-Beam Dynamical Simulation of Electron Backscatter Diffraction Patterns},
author = {Aimo Winkelmann and Carol Trager-Cowan and Francis Sweeney and Austin P. Day and Peter Parbrook},
year = {2007},
pages = {414--421},
journal = {Ultramicroscopy},
keywords = {electron backscatter diffraction, nanoscience, wave theory, Solid state physics. Nanoscience, Instrumentation, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials},
url = {http://strathprints.strath.ac.uk/3089/},
abstract = {We present an approach for the simulation of complete electron backscatter diffraction (EBSD) patterns where the relative intensity distributions in the patterns are accurately reproduced. The Bloch wave theory is applied to describe the electron diffraction process. For the simulation of experimental patterns with a large field of view, a large number of reflecting planes has to be taken into account. This is made possible by the Bethe perturbation of weak reflections. Very good agreement is obtained for simulated and experimental patterns of gallium nitride GaNf0001g at 20 kV electron energy. Experimental features like zone-axis fine structure and higher-order Laue zone rings are accurately reproduced. We discuss the influence of the diffraction of the incident beam in our experiment.}
}

Cryptoperthitic alkali feldspars within the Proterozoic Klokken syenite have been pervasively altered deuterically to patch perthites whose constituent subgrains display very fine-scale zoning in optical cathodoluminescence (CL). Electron-probe analyses and hyperspectral maps were acquired from several areas of patch perthite to potentially identify the centers responsible for CL emission and to access information that the subgrains might provide about the late-stage geological evolution of the pluton. Each hyperspectral map is composed of tens of thousands of CL spectra covering a wavelength range of 350-850 nm, and images can be formed from any desired wavelength band. The patch perthite subgrains have two emission bands: one in the blue (similar to 460 nm) and the other at red to infrared wavelengths (similar to 690-725 nm). Cathodoluminescence images formed using both bands show that the optically visible zoning is developed only at blue wavelengths. Comparisons of emission intensities with electron-probe analyses show that the blue band is activated by Ti3+ and less than similar to 25 ppm of the trace element is required for optically detectable luminescence. Adjacent Ab- and Or-rich subgrains can have identical patterns of zoning at blue wavelengths, indicating that they crystallized from the deuteric fluid simultaneously, but in most cases, differences in zoning within any one area of patch perthite indicate that subgrains grew at different times and slowly relative to the frequent changes in concentrations of Ti within the deuteric fluid. The red to infrared CL emission band is inferred to be activated by Fe3+. Ab-rich feldspar with the greatest intensities of long-wavelength emission and FeO concentrations crystallized from a late-stage and relatively low-T fluid that obtained a proportion of its Fe from mafic grains. The generally poor correlation between variations in intensities of the two emission bands within any one subgrain indicates that concentrations of Ti and Fe in the deuteric fluid varied independently of each other, and one trace element has no sensitizing or quenching effect on emission from the other center. The maximum length of zones within individual subgrains (similar to 0.5 mm) and maximum separation of subgrains with similar zoning patterns (similar to 1 mm) helps to constrain the length and interconnectivity of the thin films of deuteric fluid that mediated alteration.

@article{strathprints30535,
volume = {92},
number = {2-3},
month = {February},
author = {Martin R. Lee and Ian Parsons and Paul R. Edwards and Robert W. Martin},
title = {Identification of cathodoluminescence activators in zoned alkali feldspars by hyperspectral imaging and electron-probe microanalysis},
journal = {American Mineralogist},
pages = {243--253},
year = {2007},
keywords = {cathodoluminescence, feldspar, deuteric alteration, alkali feldspar, electron microscopy, igneous petrology, microporosity, luminescence, microscopy, dependence, chemistry, minerals, trace, Physics, Geochemistry and Petrology, Geophysics},
url = {http://strathprints.strath.ac.uk/30535/},
abstract = {Cryptoperthitic alkali feldspars within the Proterozoic Klokken syenite have been pervasively altered deuterically to patch perthites whose constituent subgrains display very fine-scale zoning in optical cathodoluminescence (CL). Electron-probe analyses and hyperspectral maps were acquired from several areas of patch perthite to potentially identify the centers responsible for CL emission and to access information that the subgrains might provide about the late-stage geological evolution of the pluton. Each hyperspectral map is composed of tens of thousands of CL spectra covering a wavelength range of 350-850 nm, and images can be formed from any desired wavelength band. The patch perthite subgrains have two emission bands: one in the blue (similar to 460 nm) and the other at red to infrared wavelengths (similar to 690-725 nm). Cathodoluminescence images formed using both bands show that the optically visible zoning is developed only at blue wavelengths. Comparisons of emission intensities with electron-probe analyses show that the blue band is activated by Ti3+ and less than similar to 25 ppm of the trace element is required for optically detectable luminescence. Adjacent Ab- and Or-rich subgrains can have identical patterns of zoning at blue wavelengths, indicating that they crystallized from the deuteric fluid simultaneously, but in most cases, differences in zoning within any one area of patch perthite indicate that subgrains grew at different times and slowly relative to the frequent changes in concentrations of Ti within the deuteric fluid. The red to infrared CL emission band is inferred to be activated by Fe3+. Ab-rich feldspar with the greatest intensities of long-wavelength emission and FeO concentrations crystallized from a late-stage and relatively low-T fluid that obtained a proportion of its Fe from mafic grains. The generally poor correlation between variations in intensities of the two emission bands within any one subgrain indicates that concentrations of Ti and Fe in the deuteric fluid varied independently of each other, and one trace element has no sensitizing or quenching effect on emission from the other center. The maximum length of zones within individual subgrains (similar to 0.5 mm) and maximum separation of subgrains with similar zoning patterns (similar to 1 mm) helps to constrain the length and interconnectivity of the thin films of deuteric fluid that mediated alteration.}
}

The usefulness of cathodoluminescence (CL) in mineralogy and petrology is largely due to its high sensitivity to variations in the chemical compositions of minerals. Difficulty in interpretation of CL images, however, often limits their use beyond the identification of growth zones, which are then analyzed using other, more readily quantifiable, techniques. The use of scanning electron microscopy (SEM) CL in hyperspectral imaging mode, combined with simultaneously acquired X-ray composition mapping, extends the technique by allowing the separation of spectral features and their correlation with elemental composition. In this paper, we describe the use of such measurements in conjunction with multivariate statistical analysis to automatically identify and characterize zones in calcite and zircon. We demonstrate that this novel combination of techniques significantly increases the effectiveness of CL as a diagnostic tool for Earth science applications.

@article{strathprints30534,
volume = {92},
number = {2-3},
month = {February},
author = {Paul R. Edwards and Robert W. Martin and Martin R. Lee},
title = {Combined cathodoluminescence hyperspectral imaging and wavelength dispersive X-ray analysis of minerals},
journal = {American Mineralogist},
pages = {235--242},
year = {2007},
keywords = {cathodoluminescence, hyperspectral, principal components, calcite, zircon, X-ray microanalysis, microscopy, Physics, Geochemistry and Petrology, Geophysics},
url = {http://strathprints.strath.ac.uk/30534/},
abstract = {The usefulness of cathodoluminescence (CL) in mineralogy and petrology is largely due to its high sensitivity to variations in the chemical compositions of minerals. Difficulty in interpretation of CL images, however, often limits their use beyond the identification of growth zones, which are then analyzed using other, more readily quantifiable, techniques. The use of scanning electron microscopy (SEM) CL in hyperspectral imaging mode, combined with simultaneously acquired X-ray composition mapping, extends the technique by allowing the separation of spectral features and their correlation with elemental composition. In this paper, we describe the use of such measurements in conjunction with multivariate statistical analysis to automatically identify and characterize zones in calcite and zircon. We demonstrate that this novel combination of techniques significantly increases the effectiveness of CL as a diagnostic tool for Earth science applications.}
}

This thematic set contains a series of papers arising from a symposium of the same name at the 15th Annual Goldschmidt Conference, held at the University of Moscow, Idaho, in May of 2005. Although luminescence (the ability of materials to emit light upon excitation with various kinds of energy) is a well-established technique in physics and the material sciences, its present widespread use in the geosciences was stimulated forty years ago by the introduction of the technique of cathodoluminescence (CL) to image the internal structure of minerals (Smith and Stenstrom 1965; Long and Agrell 1965). Many minerals exhibit the phenomenon of CL as a result of electron-beam interaction, a trait that can successfully be applied to the study and interpretation of zoning patterns in minerals and/or cementation in rocks (e.g., Mariano 1988).

@article{strathprints30533,
volume = {92},
number = {2-3},
month = {February},
author = {Ian M. Coulson and Paul R. Edwards and Martin R. Lee},
title = {Preface : recent developments in microbeam cathodoluminescence with applications to mineralogy},
journal = {American Mineralogist},
pages = {233--234},
year = {2007},
keywords = {microscopy, microbeam , cathodoluminescence, minerology, Physics, Geochemistry and Petrology, Geophysics},
url = {http://strathprints.strath.ac.uk/30533/},
abstract = {This thematic set contains a series of papers arising from a symposium of the same name at the 15th Annual Goldschmidt Conference, held at the University of Moscow, Idaho, in May of 2005. Although luminescence (the ability of materials to emit light upon excitation with various kinds of energy) is a well-established technique in physics and the material sciences, its present widespread use in the geosciences was stimulated forty years ago by the introduction of the technique of cathodoluminescence (CL) to image the internal structure of minerals (Smith and Stenstrom 1965; Long and Agrell 1965). Many minerals exhibit the phenomenon of CL as a result of electron-beam interaction, a trait that can successfully be applied to the study and interpretation of zoning patterns in minerals and/or cementation in rocks (e.g., Mariano 1988).}
}

Electron beam induced current (EBIC) characterisation can provide detailed information on the influence of crystalline defects on the diffusion and recombination of minority carriers in semiconductors. New developments are required for GaN light emitting devices, which need a cross-sectional approach to provide access to their complex multi-layered structures. A sample preparation approach based on low-voltage Ar ion milling is proposed here and shown to produce a flat cross-section with very limited surface recombination, which enables low-voltage high resolution EBIC characterisation. Dark defects are observed in EBIC images and correlation with cathodoluminescence images identify them as threading dislocations. Emphasis is placed on one-dimensional quantification which is used to show that junction delineation with very good spatial resolution can be achieved, revealing significant roughening of this GaN p-n junction. Furthermore, longer minority carrier diffusion lengths along the c-axis are found at dislocation sites, in both p-GaN and the multi-quantum well (MQW) region. This is attributed to gettering of point defects at threading dislocations in p-GaN and higher escape rate from quantum wells at dislocation sites in the MQW region, respectively. These developments show considerable promise for the use of low-voltage cross-sectional EBIC in the characterisation of point and extended defects in GaN-based devices and it is suggested that this technique will be particularly useful for degradation analysis. (c) 2006 Elsevier B.V. All rights reserved.

@article{strathprints30532,
volume = {107},
number = {4-5},
month = {May},
author = {Grigore Moldovan and Payani Kazemian and Paul R. Edwards and Vincent K. S. Ong and Oka Kurniawan and Colin J. Humphreys},
title = {Low-voltage cross-sectional EBIC for characterisation of GaN-based light emitting devices},
journal = {Ultramicroscopy},
pages = {382--389},
year = {2007},
keywords = {EBIC, GaN, diffusion, dislocations, surface recombination velocity, beam-induced-current, diffusion length, line scan, Physics, Instrumentation, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials},
url = {http://strathprints.strath.ac.uk/30532/},
abstract = {Electron beam induced current (EBIC) characterisation can provide detailed information on the influence of crystalline defects on the diffusion and recombination of minority carriers in semiconductors. New developments are required for GaN light emitting devices, which need a cross-sectional approach to provide access to their complex multi-layered structures. A sample preparation approach based on low-voltage Ar ion milling is proposed here and shown to produce a flat cross-section with very limited surface recombination, which enables low-voltage high resolution EBIC characterisation. Dark defects are observed in EBIC images and correlation with cathodoluminescence images identify them as threading dislocations. Emphasis is placed on one-dimensional quantification which is used to show that junction delineation with very good spatial resolution can be achieved, revealing significant roughening of this GaN p-n junction. Furthermore, longer minority carrier diffusion lengths along the c-axis are found at dislocation sites, in both p-GaN and the multi-quantum well (MQW) region. This is attributed to gettering of point defects at threading dislocations in p-GaN and higher escape rate from quantum wells at dislocation sites in the MQW region, respectively. These developments show considerable promise for the use of low-voltage cross-sectional EBIC in the characterisation of point and extended defects in GaN-based devices and it is suggested that this technique will be particularly useful for degradation analysis. (c) 2006 Elsevier B.V. All rights reserved.}
}

The authors report correlations between variations in charge transport of electrons and holes in synthetic single crystal diamond and the presence of nitrogen impurities and dislocations. The spatial distribution of these defects was imaged using their characteristic luminescence emission and compared with maps of carrier drift length measured by ion beam induced charge imaging. The images indicate a reduction of electron and hole mobility-lifetime product due to nitrogen impurities and dislocations. Very good charge transport is achieved in selected regions where the dislocation density is minimal.

A combined modeling and experimental analysis of InGaN deposition in the widely used single-wafer Aixtron AIX 200/4 RF-S reactor is presented. The main focus of the study is the effect of the deposition temperature on the layer composition. InGaN epilayers were grown at setpoint temperatures between 760 and 920 ?C, keeping other process parameters constant. Epilayer compositions were analysed using strainindependent methods. Results from the modeling generally match the experimental compositional data well, and thus are used to analyze factors affecting indium transport and incorporation efficiency.

This paper shows the selective etching process of an AlInN sacrificial layer, lattice-matched to GaN, on N-face GaN by an aqueous solution of 1,2-diaminoethane. Using the wavelength dispersive X-ray (WDX) spectrometers on an electron probe micro-analyser, together with an optical spectrometer and silicon CCD array added to the light microscope, and sharing the same focus as the electron microscope, cathodoluminescence spectra are collected from exactly the same spot as sampled by the WDX spectrometers. This technique allows the compositional properties of the etched AlInN layer and the optical properties of the semiconductor layers underlying the sacrificial layer to be scrutinised, verifying the etching selectivity and the efficiency of the process.

@article{strathprints9071,
volume = {40},
number = {4-6},
title = {Wavelength dispersive X-ray analysis and cathodoluminescence techniques for monitoring the chemical removal of AllnN on an n-face GaN surface},
author = {F. Rizzi and P.R. Edwards and I.M. Watson and R.W. Martin},
year = {2006},
pages = {369--372},
journal = {Superlattices and Microstructures},
keywords = {GaN, AlInN, wavelength dispersive X-ray analysis, cathodoluminescence, wet etching, Optics. Light, Physics, Materials Science(all), Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/9071/},
abstract = {This paper shows the selective etching process of an AlInN sacrificial layer, lattice-matched to GaN, on N-face GaN by an aqueous solution of 1,2-diaminoethane. Using the wavelength dispersive X-ray (WDX) spectrometers on an electron probe micro-analyser, together with an optical spectrometer and silicon CCD array added to the light microscope, and sharing the same focus as the electron microscope, cathodoluminescence spectra are collected from exactly the same spot as sampled by the WDX spectrometers. This technique allows the compositional properties of the etched AlInN layer and the optical properties of the semiconductor layers underlying the sacrificial layer to be scrutinised, verifying the etching selectivity and the efficiency of the process.}
}

InGaN/GaN multiple quantum wells (MQWs) emitting at 410-505 nm, with either 3 or 16 repeat periods, were grown on commercial GaN-on-silicon templates using metal organic vapour phase epitaxy. Spectroscopic and structural studies confirmed these MQWs are of similar quality to analogues grown on sapphire substrates. Wet etching of the silicon (111) substrates in HF-based solutions allowed the MQW structures to be converted into membranes up to 2 mm in diameter, suspended above macroscopic via holes. Such a fabrication step is attractive for the production of microcavities, and other forms of surface emitting laser. Several MQWs have been compared by photoluminescence and cathodoluminescence spectroscopy before and after conversion to membranes. These measurements indicated a consistent increase in luminescence intensity after substrate removal, accompanied by small redshifts in peak position. We have further demonstrated plasma etching of membrane structures from the underside, as will be used to optimise structures for optical pumping, and used atomic force microscopy to monitor associated changes in surface roughness.

@article{strathprints5315,
volume = {3},
number = {6},
title = {Membrane structures containing InGaN/GaN quantum wells, fabricated by wet etching of sacrificial silicon substrates},
author = {S. Park and C. Liu and E. Gu and M.D. Dawson and I.M. Watson and K. Bejtka and P.R. Edwards and R.W. Martin},
year = {2006},
pages = {1949--1952},
journal = {Physica Status Solidi C},
keywords = {quantum wells, photonics, quantum physics, optics, lasers, Optics. Light, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/5315/},
abstract = {InGaN/GaN multiple quantum wells (MQWs) emitting at 410-505 nm, with either 3 or 16 repeat periods, were grown on commercial GaN-on-silicon templates using metal organic vapour phase epitaxy. Spectroscopic and structural studies confirmed these MQWs are of similar quality to analogues grown on sapphire substrates. Wet etching of the silicon (111) substrates in HF-based solutions allowed the MQW structures to be converted into membranes up to 2 mm in diameter, suspended above macroscopic via holes. Such a fabrication step is attractive for the production of microcavities, and other forms of surface emitting laser. Several MQWs have been compared by photoluminescence and cathodoluminescence spectroscopy before and after conversion to membranes. These measurements indicated a consistent increase in luminescence intensity after substrate removal, accompanied by small redshifts in peak position. We have further demonstrated plasma etching of membrane structures from the underside, as will be used to optimise structures for optical pumping, and used atomic force microscopy to monitor associated changes in surface roughness.}
}

Gallium nitride (GaN) and related alloys are important semiconductor materials for fabricating novel photonic devices such as ultraviolet (UV) light-emitting diodes (LEDs) and vertical cavity surface-emitting lasers (VCSELs). Recent technical advances have made free-standing GaN substrates available and affordable. However, these materials are strongly resistant to wet chemical etching and also, low etch rates restrict the use of dry etching. Thus, to develop alternative high-resolution processing for these materials is increasingly important. In this paper, we report the fabrication of microstructures in free-standing GaN using pulsed UV lasers. An effective method was first developed to remove the re-deposited materials due to the laser machining. In order to achieve controllable machining and high resolution in GaN, machining parameters were carefully optimised. Under the optimised conditions, precision features such as holes (through holes, blind or tapered holes) on a tens of micrometer length scale have been machined. To fabricate micro-trenches in GaN with vertical sidewalls and a flat bottom, different process strategies of laser machining were tested and optimised. Using this technique, we have successfully fabricated high-quality micro-trenches in free-standing GaN with various widths and depths. The approach combining UV laser micromachining and other processes is also discussed. Our results demonstrate that the pulsed UV laser is a powerful tool for fabricating precision microstructures and devices in gallium nitride.

@article{strathprints5264,
volume = {252},
number = {13},
author = {E. Gu and H. Howard and A. Conneely and G.M. O'Connor and E.K. Illy and M.R.H. Knowles and P.R. Edwards and R.W. Martin and I.M. Watson and M.D. Dawson},
note = {Also available as a conference paper at http://strathprints.strath.ac.uk/9039/},
title = {Microfabrication in free-standing gallium nitride using UV laser micromachining},
journal = {Applied Surface Science},
pages = {4897--4901},
year = {2006},
keywords = {gallium nitride, pulsed UV lasers, micromachining, lasers, photonics, Optics. Light, Surfaces, Coatings and Films},
url = {http://strathprints.strath.ac.uk/5264/},
abstract = {Gallium nitride (GaN) and related alloys are important semiconductor materials for fabricating novel photonic devices such as ultraviolet (UV) light-emitting diodes (LEDs) and vertical cavity surface-emitting lasers (VCSELs). Recent technical advances have made free-standing GaN substrates available and affordable. However, these materials are strongly resistant to wet chemical etching and also, low etch rates restrict the use of dry etching. Thus, to develop alternative high-resolution processing for these materials is increasingly important. In this paper, we report the fabrication of microstructures in free-standing GaN using pulsed UV lasers. An effective method was first developed to remove the re-deposited materials due to the laser machining. In order to achieve controllable machining and high resolution in GaN, machining parameters were carefully optimised. Under the optimised conditions, precision features such as holes (through holes, blind or tapered holes) on a tens of micrometer length scale have been machined. To fabricate micro-trenches in GaN with vertical sidewalls and a flat bottom, different process strategies of laser machining were tested and optimised. Using this technique, we have successfully fabricated high-quality micro-trenches in free-standing GaN with various widths and depths. The approach combining UV laser micromachining and other processes is also discussed. Our results demonstrate that the pulsed UV laser is a powerful tool for fabricating precision microstructures and devices in gallium nitride.}
}

The effects of lattice-matched (Al,In)N insertion layers on the optical and structural properties of GaN films grown epitaxially on freestanding GaN substrates are described. The intensities and energetic positions of the GaN excitonic transitions in the photoluminescence from material grown above (Al,In)N show that high material quality and low strain have been preserved. The free exciton energies of the overgrown GaN, measured by reflectivity and photoluminescence, are 3.477, 3.482, and 3.500 eV for A, B, and C excitons, respectively, corresponding to strain-free GaN. The spectra are compared with those of a similar heterostructure grown on sapphire and of GaN-on-sapphire templates.

@article{strathprints5097,
volume = {89},
number = {191912},
month = {November},
author = {K. Bejtka and R.W. Martin and I.M. Watson and S. Ndiaye and M. Leroux},
title = {Growth and optical and structural characterization of GaN on free-standing GaN substrates with an (Al,In)N insertion layer},
journal = {Applied Physics Letters},
pages = {191912--1},
year = {2006},
keywords = {growth characterisation, optical characterisation, structural characterization, GaN, GaN substrates, photonics, nanoscience, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/5097/},
abstract = {The effects of lattice-matched (Al,In)N insertion layers on the optical and structural properties of GaN films grown epitaxially on freestanding GaN substrates are described. The intensities and energetic positions of the GaN excitonic transitions in the photoluminescence from material grown above (Al,In)N show that high material quality and low strain have been preserved. The free exciton energies of the overgrown GaN, measured by reflectivity and photoluminescence, are 3.477, 3.482, and 3.500 eV for A, B, and C excitons, respectively, corresponding to strain-free GaN. The spectra are compared with those of a similar heterostructure grown on sapphire and of GaN-on-sapphire templates.}
}

Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted AlxGa1-xN with different AlN contents (in the range 0 {\ensuremath{<}}= x {\ensuremath{<}}= 0.2) and from implanted InxAl1-xN with different InN contents (x = 0.13 and 0.19) close to the lattice match with GaN. The Tm3+ emission spectrum depends critically on the host material. The blue emission from AIxGal-xN:Tm peaks in intensity for an AlN content of x similar to 0.11. The emission is enhanced by up to a factor of 50 times with an increase of annealing temperature from 1000 to 1300 degrees C. The blue emission from In0.13Al0.87N:Tm, annealed at 1200 degrees C, is more than ten times stronger than that from AlxGa1-xN:Tm, x {\ensuremath{<}}= 0.2. However, the intensity decreases significantly as the InN fraction increases from 0.13 to 0.19.

@Article{strathprints42400,
author = {I. S. Roqan and K. Lorenz and K. P. O'Donnell and C. Trager-Cowan and R. W. Martin and I. M. Watson and E. Alves},
title = {Blue cathodoluminescence from thulium implanted AlₓGa₁₋ₓN and InₓAl₁₋ₓN},
journal = {Superlattices and Microstructures},
year = {2006},
volume = {40},
number = {4-6},
pages = {445--451},
month = {December},
abstract = {Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted AlxGa1-xN with different AlN contents (in the range 0 {\ensuremath{<}}= x {\ensuremath{<}}= 0.2) and from implanted InxAl1-xN with different InN contents (x = 0.13 and 0.19) close to the lattice match with GaN. The Tm3+ emission spectrum depends critically on the host material. The blue emission from AIxGal-xN:Tm peaks in intensity for an AlN content of x similar to 0.11. The emission is enhanced by up to a factor of 50 times with an increase of annealing temperature from 1000 to 1300 degrees C. The blue emission from In0.13Al0.87N:Tm, annealed at 1200 degrees C, is more than ten times stronger than that from AlxGa1-xN:Tm, x {\ensuremath{<}}= 0.2. However, the intensity decreases significantly as the InN fraction increases from 0.13 to 0.19.},
keywords = {rare earth, GaAlN, InAlN, implantation, cathodoluminescence, Physics, Materials Science(all), Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/42400/}
}

Monte Carlo simulations of anomalous ion channeling in near-lattice-matched AlInN/GaN bilayers allow an accurate determination of the strain state of AlInN by Rutherford backscattering or channeling. Although these strain estimates agree well with x-ray diffraction (XRD) results, XRD composition estimates are shown to have limited accuracy, due to a possible deviation from Vegard’s law, which we quantify for this alloy. As the InN fraction increases from 13\% to 19\%, the strain in AlInN films changes from tensile to compressive with lattice matching predicted to occur at [InN]=17.1\%.

AlInN layers implanted with europium and erbium ions are systematically studied and compared with similarly implanted GaN. Cathodoluminescence from four series of annealed samples shows that the Eu/Er emissions from AlInN are considerably broader than those from GaN, while the peak positions only change slightly. The rate of increase of cathodoluminescence intensity with annealing temperature, up to 1300 ?C, is analyzed for all four series. For Eu the increase exceeds 10{$\times$} in both hosts. Although some decomposition is observed for annealing at 1200 ?C, well above the growth temperature, AlInN is shown to be a surprisingly robust host for rare earth ions.

@Article{strathprints37517,
author = {K. Wang and R.W. Martin and E. Nogales and P.R. Edwards and K.P. O'Donnell and K. Lorenz and E. Alves and I.M. Watson},
title = {Cathodoluminescence of rare earth implanted AlInN},
journal = {Applied Physics Letters},
year = {2006},
volume = {89},
number = {13},
pages = {131912},
abstract = {AlInN layers implanted with europium and erbium ions are systematically studied and compared with similarly implanted GaN. Cathodoluminescence from four series of annealed samples shows that the Eu/Er emissions from AlInN are considerably broader than those from GaN, while the peak positions only change slightly. The rate of increase of cathodoluminescence intensity with annealing temperature, up to 1300 ?C, is analyzed for all four series. For Eu the increase exceeds 10{$\times$} in both hosts. Although some decomposition is observed for annealing at 1200 ?C, well above the growth temperature, AlInN is shown to be a surprisingly robust host for rare earth ions.},
keywords = {cathodoluminescence, aluminium compounds, indium compounds, gallium compounds, wide band gap semiconductors, rare earth, AllnN, implanted, Physics, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/37517/}
}

Comparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tin and GaN co-implanted with Tin and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (similar to 477 nm) which is due to intra 4f-shell electron transitions ((1)G(4)-{\ensuremath{>}} H-3(6)) associated with Tm3+ ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200 degrees C. Blue PL emission has also been observed from the sample annealed at 1200 degrees C. To our knowledge, this is the first observation of blue PL emission from Tin implanted GaN samples. Intra-4f transitions from the D-1(2) level (similar to 465 nm emission lines) of Tm3+ ions in GaN have been observed in GaN:Tm films at temperatures between 20-200 K. We will discuss the temperature dependent Tm3+ emission in both GaN:Tm,Er and GaN:Tm samples.

@incollection{strathprints36884,
month = {March},
author = {Iman Roqan and Carol Trager-Cowan and Ben Hourahine and Katharina Lorenz and Emilio Nogales and Kevin P. O'Donnell and Robert W. Martin and Eduardo Alves and S. Ruffenach and Olivier Briot},
series = {Materials Research Society Symposium Proceedings},
note = {(c) 2006 Cambridge University Press.},
booktitle = {GaN, AIN, InN and Related Materials},
editor = {M. Kuball and T. Mukai and T.H. Myers and J.M. Redwing},
title = {Characterization of the blue emission of Tm/Er co-implanted GaN},
address = {Warrendale},
publisher = {Materials Research Society},
year = {2006},
pages = {599--604},
keywords = {tm-doped alxga1-xn, photoluminescence, cathodoluminescence, ions, EU, ER , ion-implantation, nitride, luminescence, Physics, Electronic, Optical and Magnetic Materials, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/36884/},
abstract = {Comparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tin and GaN co-implanted with Tin and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (similar to 477 nm) which is due to intra 4f-shell electron transitions ((1)G(4)-{\ensuremath{>}} H-3(6)) associated with Tm3+ ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200 degrees C. Blue PL emission has also been observed from the sample annealed at 1200 degrees C. To our knowledge, this is the first observation of blue PL emission from Tin implanted GaN samples. Intra-4f transitions from the D-1(2) level (similar to 465 nm emission lines) of Tm3+ ions in GaN have been observed in GaN:Tm films at temperatures between 20-200 K. We will discuss the temperature dependent Tm3+ emission in both GaN:Tm,Er and GaN:Tm samples.}
}

In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging-in the scanning electron microscope-to study tilt, atomic steps and dislocations in epitaxial GaN thin films. We show results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02 degrees, in GaN thin films. As EBSD has a spatial resolution of approximate to 20 rim, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that channeling contrast in electron channeling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films.

@incollection{strathprints36883,
author = {Carol Trager-Cowan and Francis Sweeney and A.J. Wilkinson and P.W. Trimby and A.P. Day and A Gholinia and N.H. Schmidt and P.J. Parbrook and Ian Watson},
series = {Materials research society symposium proceedings},
booktitle = {GaN, AIN, InN and related materials},
editor = {M Kuball and T.H. Myers and J.M. Redwing and T Mukai},
address = {Warrendale},
title = {Characterization of nitride thin films by electron backscatter diffraction and electron channeling contrast imaging},
publisher = {Materials Research Society},
year = {2006},
pages = {677--682},
keywords = {plan-view image, kikuchi diffraction, GAN, dislocations, microscope, strain, rocks, SEM, Physics},
url = {http://strathprints.strath.ac.uk/36883/},
abstract = {In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging-in the scanning electron microscope-to study tilt, atomic steps and dislocations in epitaxial GaN thin films. We show results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02 degrees, in GaN thin films. As EBSD has a spatial resolution of approximate to 20 rim, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that channeling contrast in electron channeling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films.}
}

There has been substantial interest in the behaviour of hydrogen in silicon over the last decade, often focused on the behaviour of the interstitial hydrogen molecule and \{I I I\} oriented platelets. Less is known about analogous hydrogen-related defects in germanium, but planar defects are known, and the molecule has possibly been observed recently by Raman scattering. We present preliminary results of first-principles calculations on both the H-2 molecule and a range of platelet geometries ill germanium. For comparison the molecule in GaAs and Si is also simulated. Energetics and vibrational modes of the defects are presented. Our calculations show the observed weak mode at 3834cm(-1) in Ge is indeed consistent with the interstitial hydrogen molecule. (c) 2005 Elsevier B.V. All rights reserved.

@article{strathprints31165,
volume = {376-377},
month = {April},
title = {Hydrogen molecules and platelets in germanium},
author = {B. Hourahine and R. Jones and P.R. Briddon},
year = {2006},
pages = {105--108},
journal = {Physica B: Condensed Matter},
keywords = {hydrogen, germanium, theory, platelet, molecule, extended defect formation, crystlline silicon, oxygen, diffusion, Physics, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31165/},
abstract = {There has been substantial interest in the behaviour of hydrogen in silicon over the last decade, often focused on the behaviour of the interstitial hydrogen molecule and \{I I I\} oriented platelets. Less is known about analogous hydrogen-related defects in germanium, but planar defects are known, and the molecule has possibly been observed recently by Raman scattering. We present preliminary results of first-principles calculations on both the H-2 molecule and a range of platelet geometries ill germanium. For comparison the molecule in GaAs and Si is also simulated. Energetics and vibrational modes of the defects are presented. Our calculations show the observed weak mode at 3834cm(-1) in Ge is indeed consistent with the interstitial hydrogen molecule. (c) 2005 Elsevier B.V. All rights reserved.}
}

Electroluminescence from rare earth-doped nitride semiconductors shows promise for a variety of applications since strong room temperature luminescence can be observed in these materials for a variety of lanthanide dopants. Modelling of the microscopic structure and properties of the defects involved in the luminescence presents a substantial challenge to current theoretical methods. While it is possible to investigate issues of defect stability using pseudopotential-based approaches, which avoid the problems of modelling strongly correlated f-electron systems, this cannot address luminescence from these centres. Explicitly treating 4f electrons is beyond the reach of the usual mean-field methods normally employed in density-functional theory. In an attempt to improve the theoretical description of these systems while extending the size of models used, we present the results using density-functional -based tight-binding calculations on the properties of erbium in wurtzite GaN. Both substitutional defects and complexes with nitrogen vacancies are considered. We account for strong correlation of the 4f shell using a variant of the LDA+ U method. (c) 2005 Elsevier B.V. All rights reserved.

@article{strathprints31164,
volume = {376-377},
month = {April},
title = {A theoretical study of erbium in GaN},
author = {B. Hourahine and S. Sanna and B. Aradi and C. Kohler and Th. Frauenheim},
year = {2006},
pages = {512--515},
journal = {Physica B: Condensed Matter},
keywords = {GaN, erbium, theory, TB-Doped GaN, ER, films, EU, MBE, Physics, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31164/},
abstract = {Electroluminescence from rare earth-doped nitride semiconductors shows promise for a variety of applications since strong room temperature luminescence can be observed in these materials for a variety of lanthanide dopants. Modelling of the microscopic structure and properties of the defects involved in the luminescence presents a substantial challenge to current theoretical methods. While it is possible to investigate issues of defect stability using pseudopotential-based approaches, which avoid the problems of modelling strongly correlated f-electron systems, this cannot address luminescence from these centres. Explicitly treating 4f electrons is beyond the reach of the usual mean-field methods normally employed in density-functional theory. In an attempt to improve the theoretical description of these systems while extending the size of models used, we present the results using density-functional -based tight-binding calculations on the properties of erbium in wurtzite GaN. Both substitutional defects and complexes with nitrogen vacancies are considered. We account for strong correlation of the 4f shell using a variant of the LDA+ U method. (c) 2005 Elsevier B.V. All rights reserved.}
}

Rare-earth (RE) doped III-nitrides, prepared by in-situ doping during growth or by ion implantation and annealing, are promising materials for visible light emitting displays. In addition, they are plantation extremely challenging theoretically, on account of the complexity of the sharp inter-4f optical transitions, which are allowed only through the mixing by non-centrosymmetric crystal. elds of the inner-4f orbitals with higher-lying states of opposite parity. We review recent experimental and theoretical work on Er-, Eu- and Tm-doped III-nitride compounds and alloys which has been carried out with a view to establishing the lattice location of RE in these materials and the probable nanostructure of the centres which are responsible for their luminescence. The isolated site REIII is found to be both optically and electrically inactive, but in association with neighbouring intrinsic defects (most probably nitrogen vacancies) REIII can generate a small family of similar optically active sites. Such a family is held to be responsible for the site multiplicity that is a common feature of the spectroscopy of RE-doped III-nitrides.

@article{strathprints31161,
volume = {36},
number = {2},
month = {November},
author = {K.P. O'Donnell and B. Hourahine},
title = {Rare earth doped III-nitrides for optoelectronics},
journal = {European Physical Journal: Applied Physics},
pages = {91--103},
year = {2006},
keywords = {EU implanted GaN, spectra, optical properties, electrical activity, fine structure, ER, erbium, photoluminescence, excitation, Physics, Instrumentation, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31161/},
abstract = {Rare-earth (RE) doped III-nitrides, prepared by in-situ doping during growth or by ion implantation and annealing, are promising materials for visible light emitting displays. In addition, they are plantation extremely challenging theoretically, on account of the complexity of the sharp inter-4f optical transitions, which are allowed only through the mixing by non-centrosymmetric crystal. elds of the inner-4f orbitals with higher-lying states of opposite parity. We review recent experimental and theoretical work on Er-, Eu- and Tm-doped III-nitride compounds and alloys which has been carried out with a view to establishing the lattice location of RE in these materials and the probable nanostructure of the centres which are responsible for their luminescence. The isolated site REIII is found to be both optically and electrically inactive, but in association with neighbouring intrinsic defects (most probably nitrogen vacancies) REIII can generate a small family of similar optically active sites. Such a family is held to be responsible for the site multiplicity that is a common feature of the spectroscopy of RE-doped III-nitrides.}
}

Lattice recovery of Eu-implanted GaN has been studied by means of Raman scattering under UV excitation. GaN epilayers implanted at 300 keV with doses ranging from 2 x 10(14) to 4 x 10(15) cm(-2) and subsequently annealed at 1000 degrees C for 20 min show an increasing degree of disorder as the implantation dose increases. Higher temperature annealings up to 1300 degrees C were also investigated in samples having an AlN capping layer. Disorder related modes, observed in samples annealed at 1000 degrees C, disappear at higher annealing temperatures, indicating an incomplete lattice recovery at 1000 degrees C. The Raman scattering spectra show resonant A(1)(LO) multiphonon scattering up to sixth order, whose relative intensities depend on the implantation dose. The intensity ratios between multiphonon peaks observed for the highest implantation doses suggest a spread of the resonance, which could be related to a heterogeneous strain distribution, also indicative of incomplete lattice recovery. (c) 2006 Elsevier Ltd. All rights reserved.

A 10 nm thick epitaxially grown AIN cap has been used to protect the surface of a GaN epilayer both during Eu ion implantation and the subsequent high-temperature annealing. The 15 K photoluminescence (PL) intensity of the intra-4f Eu transition increases by two orders of magnitude when the annealing temperature is increased from 1000 to 1300 degrees C. High-resolution PL spectra reveal that the emission lines due to the D-5(0)-F-7(2) transition exhibit different dependencies on the annealing temperature in the studied annealing range. PL excitation measurements demonstrate band edge absorption by the GaN host at 356 nm, together with a broad excitation band centred at similar to 385 nm. The PL spectra of the D-5(0)-F-7(2) transition selectively excited by above band-gap absorption and by this broad excitation band are noticeably different. The first peak at 620.8 nm is suppressed when exciting below the GaN band gap. This demonstrates differing energy transfer processes for the different Eu luminescent peaks and is direct evidence for at least two kinds of different Eu sites in the host with distinct optical activation. Temperature dependent PL and PLE demonstrate that one of the two Eu-centres does not contribute to the room temperature luminescence. (c) 2005 Elsevier B.V. All rights reserved.

@article{strathprints31052,
volume = {28},
number = {6-7},
month = {May},
author = {K Wang and RW Martin and E Nogales and V Katchkanov and KP O'Donnell and S Hernandez and K Lorenz and E Alves and S Ruffenach and O Briot},
note = {Meeting of the European-Materials-Research-Society, Strasbourg, FRANCE, MAY 30-JUN 03, 2005},
title = {Optical properties of high-temperature annealed Eu-implanted GaN},
year = {2006},
journal = {Optical Materials},
pages = {797--801},
keywords = {photoluminescence , luminescence, optics, lasers, beams, Optics. Light, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics, Electrical and Electronic Engineering, Computer Science(all)},
url = {http://strathprints.strath.ac.uk/31052/},
abstract = {A 10 nm thick epitaxially grown AIN cap has been used to protect the surface of a GaN epilayer both during Eu ion implantation and the subsequent high-temperature annealing. The 15 K photoluminescence (PL) intensity of the intra-4f Eu transition increases by two orders of magnitude when the annealing temperature is increased from 1000 to 1300 degrees C. High-resolution PL spectra reveal that the emission lines due to the D-5(0)-F-7(2) transition exhibit different dependencies on the annealing temperature in the studied annealing range. PL excitation measurements demonstrate band edge absorption by the GaN host at 356 nm, together with a broad excitation band centred at similar to 385 nm. The PL spectra of the D-5(0)-F-7(2) transition selectively excited by above band-gap absorption and by this broad excitation band are noticeably different. The first peak at 620.8 nm is suppressed when exciting below the GaN band gap. This demonstrates differing energy transfer processes for the different Eu luminescent peaks and is direct evidence for at least two kinds of different Eu sites in the host with distinct optical activation. Temperature dependent PL and PLE demonstrate that one of the two Eu-centres does not contribute to the room temperature luminescence. (c) 2005 Elsevier B.V. All rights reserved.}
}

The structural properties of Er doped GaN epilayers were studied by means of extended X-ray absorption fine structure (EXAFS) measured at the Er L-III and Ga K-edges. The samples were doped with Er in-situ during growth by molecular beam epitaxy (MBE). The Ga local structure was found to be the same in all samples studied. Er L-III-edge EXAFS showed that when growth conditions were gradually changed from Ga-rich to Ga-poor, an increase in Er concentration from 0.15 at.\% to 0.64 at.\% is accompanied by the sequential formation of ErGaN, ErGaN clusters with locally high Er content and finally a pure ErN component. This study indicates that Er incorporation into GaN is enhanced under Ga-poor conditions, at the expense of the formation of Er-rich clusters and ErN precipitates. (c) 2005 Elsevier B.V. All rights reserved.

We have investigated the crystalline quality of thulium-doped GaN obtained either by in situ doping during MBE growth or by ion-beam implantation of MOCVD GaN layers. Both types of samples display the typical sharp intra-4f shell emission lines of Tm3+ ions in the blue and infrared spectral regions. The Raman spectra of the MBE samples indicate a good crystalline quality, showing the narrow E-2 and A(1) phonon peaks characteristic of GaN, even for the highest Tin concentrations. In contrast, Raman peaks associated with vacancy-related defects, as well as low-frequency bands due to disorder-activated modes can be observed in the Raman spectra of the implanted samples. These results indicate that, for the implantation doses required to achieve Tm emission, some residual disorder remains in the implanted GaN layers which is not observed in Tm-doped MBE samples with higher Tm concentration.

GaN epilayers grown by MOCVD were implanted with Tm and Eu under different implantation conditions, in order to optimize the implantation parameters of fluence, implantation temperature and energy. Rutherford backscattering spectrometry in the channelling mode (RBS/C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL) and room temperature cathodoluminescence (CL) were used to study structural and optical properties of the samples. Different annealing methods are compared. According to RBS/C measurements, Tm at low fluences is found to incorporate entirely on substitutional Ga-sites while for higher fluences the substitutional fraction decreases. Increasing the fluence leads to increasing damage levels and finally to the formation of a partially amorphous, nanocrystalline surface layer. The luminescence intensity increases with fluence at first. However., for fluences in excess of 2 x 10(15) Tm/cm(2) it decreases again, showing a strong correlation between structural and optical properties. Implanting at higher temperature inhibits the formation of the nanocrystalline layer and increases both the substitutional fraction and the luminescence intensity. We also found that the presence of a thin AIN cap protects the sample during post-implant high temperature annealing and prevents the formation of a nanocrystalline surface layer during the implantation even for high fluences. The intensity of Eu-related CL near 622 nm at room temperature increases by a factor of 40 within the studied annealing range from 1000 to 1300 degrees C. (c) 2005 Elsevier B.V. All rights reserved.

@article{strathprints31049,
volume = {28},
number = {6-7},
month = {May},
author = {K. Lorenz and U. Wahl and E. Alves and E Nogales and S Dalmasso and R W Martin and K P O'Donnell and M Wojdak and A Braud and T Monteiro and T Wojtowicz and P Ruterana and S Ruffenach and O Briot},
note = {Meeting of the European-Materials-Research-Society, Strasbourg, FRANCE, MAY 30-JUN 03, 2005},
title = {High temperature annealing of rare earth implanted GaN films : structural and optical properties},
year = {2006},
journal = {Optical Materials},
pages = {750--758},
keywords = {GaN films , electron microscopy , nanocrystalline surface , backscattering , spectrometry, Optics. Light, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics, Electrical and Electronic Engineering, Computer Science(all)},
url = {http://strathprints.strath.ac.uk/31049/},
abstract = {GaN epilayers grown by MOCVD were implanted with Tm and Eu under different implantation conditions, in order to optimize the implantation parameters of fluence, implantation temperature and energy. Rutherford backscattering spectrometry in the channelling mode (RBS/C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL) and room temperature cathodoluminescence (CL) were used to study structural and optical properties of the samples. Different annealing methods are compared. According to RBS/C measurements, Tm at low fluences is found to incorporate entirely on substitutional Ga-sites while for higher fluences the substitutional fraction decreases. Increasing the fluence leads to increasing damage levels and finally to the formation of a partially amorphous, nanocrystalline surface layer. The luminescence intensity increases with fluence at first. However., for fluences in excess of 2 x 10(15) Tm/cm(2) it decreases again, showing a strong correlation between structural and optical properties. Implanting at higher temperature inhibits the formation of the nanocrystalline layer and increases both the substitutional fraction and the luminescence intensity. We also found that the presence of a thin AIN cap protects the sample during post-implant high temperature annealing and prevents the formation of a nanocrystalline surface layer during the implantation even for high fluences. The intensity of Eu-related CL near 622 nm at room temperature increases by a factor of 40 within the studied annealing range from 1000 to 1300 degrees C. (c) 2005 Elsevier B.V. All rights reserved.}
}

Comparative Ga and In K-edge extended x-ray absorption fine structure studies provide the first direct evidence of an inequality of mean In-Ga and Ga-In next-nearest neighbor separations in InGaN alloys. The degree of inequality increases with decreasing InN fraction x in the range accessible to extended x-ray absorption fine structure analysis of alloys (0.9 {\ensuremath{<}}x {\ensuremath{<}}0.1). Its concurrence with an increase of luminescence efficiency in this composition range suggests that the breakdown of In/Ga randomness in InGaN is correlated with efficient radiative recombination in blue-green light emitting devices.

We have studied the structural and optical properties of InxAl1-xN alloys with compositions nearly lattice-matched to GaN. Scanning electron microscopy measurements reveals a good overall surface quality, with some defect structures distributed across the surface whose density increases with the InN concentration. On the other hand, Raman scattering experiments show three peaks in the frequency range between 500 and 900 cm which have been assigned to InN-like and AIN-like E-2 modes and A(1)(LO) mode of the InxAl1-xN. These results agree with theoretical calculations previously reported where two-mode and one-mode behavior was predicted for the E-2 and A(1)(LO) modes, respectively. Photoluminescence and photoluminescence excitation allowed us to determine the emission and absorption energies of the InxAl1-xN epilayers. Both energies display a redshift as the InN fraction increases. We find a roughly linear increase of the Stokes shift with InN fraction, with Stokes shift values of approximate to 0.5 eV in the composition range close to the lattice-matched condition.

@inproceedings{strathprints31045,
author = {S. Hernandez and K. Wang and D. Amabile and E. Nogales and D. Pastor and R. Cusco and L. Artus and R. W. Martin and K. P. O'Donnell and I. M. Watson and RENiBE1 Network},
note = {Symposium on GaN, AIN, InN Related Materials held at the 2005 MRS Fall Meeting, Boston, MA, NOV 28-DEC 02, 2005},
booktitle = {Symposium on GaN, AIN, InN Related Materials},
editor = {M Kuball and TH Myers and JM Redwing and T Mukai},
title = {Structural and optical properties of MOCVD InAlN epilayers},
journal = {Symposium on GaN, AIN, InN Related Materials},
pages = {557--562},
year = {2006},
keywords = {MOCVD InAlN epilayers, InxAl1-xN alloys , GaN, raman scattering, Physics},
url = {http://strathprints.strath.ac.uk/31045/},
abstract = {We have studied the structural and optical properties of InxAl1-xN alloys with compositions nearly lattice-matched to GaN. Scanning electron microscopy measurements reveals a good overall surface quality, with some defect structures distributed across the surface whose density increases with the InN concentration. On the other hand, Raman scattering experiments show three peaks in the frequency range between 500 and 900 cm which have been assigned to InN-like and AIN-like E-2 modes and A(1)(LO) mode of the InxAl1-xN. These results agree with theoretical calculations previously reported where two-mode and one-mode behavior was predicted for the E-2 and A(1)(LO) modes, respectively. Photoluminescence and photoluminescence excitation allowed us to determine the emission and absorption energies of the InxAl1-xN epilayers. Both energies display a redshift as the InN fraction increases. We find a roughly linear increase of the Stokes shift with InN fraction, with Stokes shift values of approximate to 0.5 eV in the composition range close to the lattice-matched condition.}
}

In the present paper the authors describe the use of electron backscatter diffraction (EBSD) mapping and electron channelling contrast imaging (in the scanning electron microscope) to study tilt, strain, atomic steps and dislocations in epitaxial GaN thin films. Results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film are shown. From the results it is deduced that EBSD may be used to measure orientation changes of the order of 0?02? and strain changes of order 2 {$\times$} 10?4 in GaN thin films. It is also demonstrated that channelling contrast in electron channelling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films. In addition the authors will consider the results of the first many-beam dynamical simulations of EBSD patterns from GaN thin films, in which the intensity distributions in the experimental patterns are accurately reproduced.

@Article{strathprints3084,
author = {C. Trager-Cowan and F. Sweeney and A. Winkelmann and A.J. Wilkinson and P.W. Trimby and A.P. Day and A. Gholinia and N.H. Schmidt and P.J. Parbrook and I.M. Watson},
title = {Characterisation of nitride thin films by electron backscatter diffraction and electron channelling contrast imaging},
journal = {Materials Science and Technology},
year = {2006},
volume = {22},
number = {11},
pages = {1352--1358},
month = {November},
abstract = {In the present paper the authors describe the use of electron backscatter diffraction (EBSD) mapping and electron channelling contrast imaging (in the scanning electron microscope) to study tilt, strain, atomic steps and dislocations in epitaxial GaN thin films. Results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film are shown. From the results it is deduced that EBSD may be used to measure orientation changes of the order of 0?02? and strain changes of order 2 {$\times$} 10?4 in GaN thin films. It is also demonstrated that channelling contrast in electron channelling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films. In addition the authors will consider the results of the first many-beam dynamical simulations of EBSD patterns from GaN thin films, in which the intensity distributions in the experimental patterns are accurately reproduced.},
keywords = {nitride thin films, electron backscatter diffraction, electron channelling, contrast imaging, nanoscience, Solid state physics. Nanoscience, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/3084/}
}

Optical cathodoluminescence (CL) is commonly used to identify diagenetically altered carbonate fossils, yet such an interpretation is problematic as present-day carbonate shells may also luminesce. Hyperspectral CL imaging combines CL microscopy and CL spectroscopy to quantitatively analyze luminescence emission. Cold optical CL and hyperspectral CL imaging were carried out on four modern biominerals, a Rhynchonelliform brachiopod, a Craniid brachiopod, a bivalve, and the eggshell of the domestic fowl. A fossil Craniid brachiopod was analyzed to compare luminescence emission with that from the modern Craniid brachiopod. The beam conditions used for optical CL vary between studies, which hinders the direct comparison of CL analyses. This study assesses the effect of beam current and beam diameter on the intensity of luminescence emission. By characterizing the effect of beam conditions on different CaCO3 biominerals, comparisons can be made between CL studies. Hyperspectral CL imaging can be carried out in combination with WDS element analysis. By comparing hyperspectral CL images with element maps the causes of luminescence can to some extent be determined. The intensity of luminescence emitted from the modern biominerals differs under the same beam conditions. All four modern shells emit blue luminescence. In N. anomala, there is a correlation between Mn2+ concentration and luminescence intensity in the 620- to 630-nm wavelength band, which is apparent in the inner region of the shell. The fossil Craniid also emits blue luminescence, and texture within the shell wall is apparent; however, the luminescence emission between 620 and 630 nm that is evident in N. anomala is absent.

@Article{strathprints30544,
author = {Jennifer England and Maggie Cusack and Niall W. Paterson and Paul Edwards and Martin R. Lee and Robert Martin},
title = {Hyperspectral cathodoluminescence imaging of modern and fossil carbonate shells},
journal = {Journal of Geophysical Research Atmospheres},
year = {2006},
volume = {111},
pages = {G03001},
month = {July},
abstract = {Optical cathodoluminescence (CL) is commonly used to identify diagenetically altered carbonate fossils, yet such an interpretation is problematic as present-day carbonate shells may also luminesce. Hyperspectral CL imaging combines CL microscopy and CL spectroscopy to quantitatively analyze luminescence emission. Cold optical CL and hyperspectral CL imaging were carried out on four modern biominerals, a Rhynchonelliform brachiopod, a Craniid brachiopod, a bivalve, and the eggshell of the domestic fowl. A fossil Craniid brachiopod was analyzed to compare luminescence emission with that from the modern Craniid brachiopod. The beam conditions used for optical CL vary between studies, which hinders the direct comparison of CL analyses. This study assesses the effect of beam current and beam diameter on the intensity of luminescence emission. By characterizing the effect of beam conditions on different CaCO3 biominerals, comparisons can be made between CL studies. Hyperspectral CL imaging can be carried out in combination with WDS element analysis. By comparing hyperspectral CL images with element maps the causes of luminescence can to some extent be determined. The intensity of luminescence emitted from the modern biominerals differs under the same beam conditions. All four modern shells emit blue luminescence. In N. anomala, there is a correlation between Mn2+ concentration and luminescence intensity in the 620- to 630-nm wavelength band, which is apparent in the inner region of the shell. The fossil Craniid also emits blue luminescence, and texture within the shell wall is apparent; however, the luminescence emission between 620 and 630 nm that is evident in N. anomala is absent.},
keywords = {recent biogenic carbonates, ontogenic fingerprint, Brachiopod shellsl , seawater, Delta-O-18, Delta-C-13, evolution, patterns, calcite, Physics, Earth and Planetary Sciences (miscellaneous), Geophysics, Atmospheric Science, Space and Planetary Science},
url = {http://strathprints.strath.ac.uk/30544/}
}

The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300 ?C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1-2 m which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.

@Article{strathprints3008,
author = {E. Nogales and R.W. Martin and K.P. O'Donnell and K. Lorenz and E. Alves and S. Ruffenach and O. Briot},
title = {Failure mechanism of AlN nanocaps used to protect RE-implanted GaN during high temperature annealing},
journal = {Applied Physics Letters},
year = {2006},
volume = {88},
pages = {31902},
month = {January},
abstract = {The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300 ?C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1-2 m which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.},
keywords = {failure mechanism, AlN nanocaps, RE-implanted GaN, high temperature annealing, nanoscience, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/3008/}
}

Variations in thickness of the GaN caps above single InGaN quantum wells have been studied using photoluminescence spectroscopy. Data are presented from two series of samples designed to promote energy transfer to luminescent species on the surface. Improvements in the optical properties as the GaN cap thickness increases from 2.5 to 15 nm are accompanied by clear changes in the intensity of the LO-phonon satellites. Analysis of the strength of successive phonon satellites and the associated Huang-Rhys factors indicates that the amount of localization of the excitons is increased for the thinner cap samples. Surface depletion fields are also considered.

@Article{strathprints10021,
author = {L.T. Tan and R.W. Martin and K.P. O'Donnell and I.M. Watson},
title = {Photoluminescence and phonon satellites of single InGaN/GaN quantum wells with varying GaN cap thickness},
journal = {Applied Physics Letters},
year = {2006},
volume = {89},
number = {10},
pages = {101910},
month = {September},
abstract = {Variations in thickness of the GaN caps above single InGaN quantum wells have been studied using photoluminescence spectroscopy. Data are presented from two series of samples designed to promote energy transfer to luminescent species on the surface. Improvements in the optical properties as the GaN cap thickness increases from 2.5 to 15 nm are accompanied by clear changes in the intensity of the LO-phonon satellites. Analysis of the strength of successive phonon satellites and the associated Huang-Rhys factors indicates that the amount of localization of the excitons is increased for the thinner cap samples. Surface depletion fields are also considered.},
keywords = {indium compounds , phonon-exciton interactions, photoluminescence, semiconductor quantum wells, III-V semiconductors, gallium compounds, Physics, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/10021/}
}

We present a study of the time-integrated and time-resolved photoluminescence properties of single-InGaN/GaN quantum dots (QDs) using two-photon spectroscopy. Two samples containing QDs produced by different growth techniques are examined. We find that two-photon excitation results in the suppression of the emission from the underlying quantum well to which the QDs are coupled and yet relatively strong QD emission is observed. This effect is explained in terms of the enhancement of two-photon absorption in QDs due to the full confinement of carriers. Furthermore, evidence of the presence of excited states is revealed from the two-photon photoluminescence excitation spectra presented in the study.

@article{strathprints10020,
volume = {32},
number = {1-2},
title = {Two-photon absorption from single InGaN/GaN quantum dots},
author = {A.F. Jarjour and A.M. Green and T.J. Parker and R.A. Taylor and R.A. Oliver and G.A.D. Briggs and M.J. Kappers and C.J. Humphreys and R.W. Martin and I.M. Watson},
year = {2006},
pages = {119--122},
journal = {Physica E: Low-dimensional Systems and Nanostructures},
keywords = {InGaN, quantum dot, two-photon absorption, photoluminescence, time-resolved, photoluminescence excitation, Physics, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/10020/},
abstract = {We present a study of the time-integrated and time-resolved photoluminescence properties of single-InGaN/GaN quantum dots (QDs) using two-photon spectroscopy. Two samples containing QDs produced by different growth techniques are examined. We find that two-photon excitation results in the suppression of the emission from the underlying quantum well to which the QDs are coupled and yet relatively strong QD emission is observed. This effect is explained in terms of the enhancement of two-photon absorption in QDs due to the full confinement of carriers. Furthermore, evidence of the presence of excited states is revealed from the two-photon photoluminescence excitation spectra presented in the study.}
}

The local structure around In atoms in InGaN epilayers grown by Molecular Beam Epitaxy (MBE) and by Metal-Organic Chemical Vapour Deposition (MOCVD) was studied by means of Extended X-ray Absorption Fine Structure (EXAFS). The averaged In fraction of MOCVD grown samples ranged from 10\% to 40\% as estimated by Electron Probe Microanalysis (EPMA). The In fraction of MBE grown samples spanned the range from 13\% to 96\%. The In-N bond length was found to vary slightly with composition, both for MBE and MOCVD grown samples. Moreover, for the same In content, the In-N bond lengths in MOCVD samples were longer than those in MBE grown samples. In contrast, the In-In radial separations in MOCVD and MBE samples were found to be indistinguishable for the same In molar fraction. The In-Ga bond length was observed to deviate from average cation-cation distance predicted by Vegard’s law for MBE grown samples which indicates alloy compositional fluctuations.

@article{strathprints9984,
volume = {831},
title = {Extended x-ray absorption fine structure studies of InGaN epilayers},
author = {V. Katchkanov and K.P. O'Donnell and J.F.W. Mosselmans and S. Hernandez and R.W. Martin and Y. Nanishi and M. kurochi and I.M. Watson and W. van der Stricht and E. Calleja},
year = {2005},
pages = {203--207},
journal = {MRS Online Proceedings Library},
keywords = {physics, materials research, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/9984/},
abstract = {The local structure around In atoms in InGaN epilayers grown by Molecular Beam Epitaxy (MBE) and by Metal-Organic Chemical Vapour Deposition (MOCVD) was studied by means of Extended X-ray Absorption Fine Structure (EXAFS). The averaged In fraction of MOCVD grown samples ranged from 10\% to 40\% as estimated by Electron Probe Microanalysis (EPMA). The In fraction of MBE grown samples spanned the range from 13\% to 96\%. The In-N bond length was found to vary slightly with composition, both for MBE and MOCVD grown samples. Moreover, for the same In content, the In-N bond lengths in MOCVD samples were longer than those in MBE grown samples. In contrast, the In-In radial separations in MOCVD and MBE samples were found to be indistinguishable for the same In molar fraction. The In-Ga bond length was observed to deviate from average cation-cation distance predicted by Vegard's law for MBE grown samples which indicates alloy compositional fluctuations.}
}

We present micro-photoluminescence measurements on single site-controlled InGaN/GaN quantum dots using two-photon excitation Furthermore, measurements of photoluminescence excitation and time-resolved photoluminescence are also presented. We show that two-photon excitation results in total suppression of the emission from the underlying quantum well, to which the quantum dots are couple, and yet strong quantum dot emission. We attribute this effect to the enhancement of the two-photon absorption in the quantum dots as a result of the zero-dimensional confinement compared to that of the quantum wells.

@article{strathprints9983,
volume = {2},
number = {11},
month = {November},
author = {A.F. Jarjour and R.A. Taylor and R.W. Martin and I.M. Watson},
title = {Two-photon absorption in site-controlled InGaN/GaN quantum dots},
journal = {Physica Status Solidi C},
pages = {3843--3846},
year = {2005},
keywords = {42.65.?k, 78.67.Hc, 78.47.+p;78.55.Cr, Physics, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/9983/},
abstract = {We present micro-photoluminescence measurements on single site-controlled InGaN/GaN quantum dots using two-photon excitation Furthermore, measurements of photoluminescence excitation and time-resolved photoluminescence are also presented. We show that two-photon excitation results in total suppression of the emission from the underlying quantum well, to which the quantum dots are couple, and yet strong quantum dot emission. We attribute this effect to the enhancement of the two-photon absorption in the quantum dots as a result of the zero-dimensional confinement compared to that of the quantum wells.}
}

This speech was presented to the 2005 Annual Conference of the British Association for Crystal Growth, held in Sheffield on Sunday 4 – Tuesday 6 September 2005. The presentation focused on the design and growth of microcavities and the roles of AlInN layer in post-growth processing.

A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated.

Angular-dependent reflection measurements were used to map out the dispersion relations of the resonant Bloch modes, and the angular dispersion of the phonon modes, in a photonic crystal consisting of selectively grown GaN pyramids containing a single InxGa1-xN/GaN quantum well. The dispersion of the photons exhibits strong photonic crystal characteristics, while the mean indium content of the 2 nm thickness InxGa1-xN layer was extracted from the angular dispersion of the phonon modes.

When lattice matched to GaN, the AlInN ternary alloy has a refractive index {\texttt{\char126}}7\% lower than that of GaN. This characteristic can be exploited to perform in situ reflectometry during epitaxial growth of GaN-based multilayer structures on free-standing GaN substrates, by insertion of a suitable Al0.82In0.18N layer. The real-time information on growth rates and cumulative layer thicknesses thus obtainable is particularly valuable in the growth of optical resonant cavity structures. We illustrate this capability with reference to the growth of InGaN/GaN multiple quantum-well structures, including a doubly periodic structure with relatively thick GaN spacer layers between groups of wells. Al0.82In0.18N insertion layers can also assist in the fabrication of resonant cavity structures in postgrowth processing, for example, acting as sacrificial layers in a lift-off process exploiting etch selectivity between Al0.82In0.18N and GaN.

The envisaged upgrade of the Large Hadron Collider (LHC) at CERN towards the Super-LHC (SLHC) with a 10 times increased luminosity of 1035 cm?2 s?1 will present severe challenges for the tracking detectors of the SLHC experiments. Unprecedented high radiation levels and track densities and a reduced bunch crossing time in the order of 10 ns as well as the need for cost effective detectors have called for an intensive R&D program. The CERN RD50 collaboration ?Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders? is working on the development of semiconductor sensors matching the requirements of the SLHC. Sensors based on defect engineered silicon like Czochralski, epitaxial and oxygen enriched silicon have been developed. With 3D, Semi-3D and thin detectors new detector concepts have been evaluated and a study on the use of standard and oxygen enriched p-type silicon detectors revealed a promising approach for radiation tolerant cost effective devices. These and other most recent advancements of the RD50 collaboration are presented.

@article{strathprints37547,
volume = {546},
number = {1-2},
month = {July},
author = {M Moll and J Adey and A Al-Ajili and G Alfieri and PP Allport and M Artuso and S Assouak and BS Avset and L Barabash and A Barcz and R Bates and SF Biagi and GM Bilei and D Bisello and A Blue and A Blumenau and V Boisvert and G Bolla and G Bondarenko and E Borchi and L Borrello and D Bortoletto and M Boscardin and L Bosisio and TJV Bowcock and TJ Brodbeck and J Broz and M Bruzzi and A Brzozowski and M Buda and P Buhmann and C Buttar and F Campabadal and D Campbell and A Candelori and G Casse and A Cavallini and S Charron and A Chilingarov and D Chren and V Cindro and P Collins and R Coluccia and D Contarato and J Coutinho and D Creanza and W Cunningham and GF Dalla Betta and I Dawson and W de Boer and M De Palma and R Demina and P Dervan and S Dittongo and Z Dolezal and A Dolgolenko and T Eberlein and V Eremin and C Fall and F Fasolo and F Fizzotti and C Fleta and E Focardi and E Forton and E Fretwurst and C Garcia and JE Garcia-Navarro and E Gaubas and MH Genest and KA Gill and K Giolo and M Glaser and C Goessling and V Golovine and SG Sevilla and I Gorelov and J Goss and AG Bates and G Gregoire and P Gregori and E Grigoriev and AA Grillo and A Groza and J Guskov and L Haddad and J Harkonen and F Hauler and M Hoeferkamp and F Honniger and T Horazdovsky and R Horisberger and M Horn and A Houdayer and Benjamin Hourahine and G Hughes and I Ilyashenko and K Irmscher and A Ivanov and K Jarasiunas and KMH Johansen and BK Jones and R Jones and C Joram and L Jungermann and E Kalinina and P Kaminski and A Karpenko and A Karpov and V Kazlauskiene and V Kazukauskas and V Khivrich and V Khomenkov and J Kierstead and J Klaiber-Lodewigs and R Klingenberga and P Kodys and Z Kohout and S Korjenevski and M Koski and R Kozlowski and M Kozodaev and G Kramberger and O Krasel and A Kuznetsov and S Kwan and S Lagomarsino and K Lassila-Perini and V Lastovetsky and G Latino and S Lazanu and I Lazanu and A Lebedev and C Lebel and K Leinonen and C Leroy and Z Li and G Lindstrom and V Linhart and A Litovchenko and P Litovchenko and A Lo Giudice and M Lozano and Z Luczynski and P Luukka and A Macchiolo and LF Makarenko and I Mandic and C Manfredotti and N Manna and S marti garcia and S Marunko and K Mathieson and J Melone and D Menichelli and A Messineo and J Metcalfe and S Miglio and M Mikuz and J Miyamoto and E Monakhov and F Moscatelli and D Naoumov and E Nossarzewska-Orlowska and J Nysten and P Olivera and V OShea and T Palvialnen and C Paolini and C Parkes and D Passeri and U Pein and G Pellegrini and L Perera and K Petasecca and C Piemonte and GU Pignatel and N Pinho and I Pintilie and L Pintilie and L Polivtsev and P Polozov and A Popa and J Popule and S Pospisil and A Pozza and V Radicci and JM Rafi and R Rando and R Roeder and T Rohe and S Ronchin and C Rott and A Roy and A Ruzin and HFW Sadrozinski and S Sakalauskas and M Scaringella and L Schiavulli and S Schnetzer and B Schumm and S Sciortino and A Scorzoni and G Segneri and S Seidel and A Seiden and G Sellberg and P Sellin and D Sentenac and I Shipsey and P Sicho and T Sloan and M Solar and S Son and B Sopko and V Sopko and N Spencer and J Stahl and D Stolze and R Stone and J Storasta and N Strokan and M Sudzius and B Surma and A Suvorov and BG Svensson and P Tipton and M Tomasek and A Tsvetkov and E Tuominen and E Tuovinen and T Tuuva and M Tylchin and H Uebersee and J Uher and M Ullan and JV Vaitkus and J Velthuis and E Verbitskaya and V Vrba and G Wagner and I Wilhelm and S Worm and V Wright and R Wunstorf and Y Yiuri and P Zabierowski and A Zaluzhny and M Zavrtanik and M Zen and V Zhukov and N Zorzi and Benjamin Hourahine},
note = {6th International Workshop on Radiation Imaging Detectors, Univ Glasgow, Glasgow, SCOTLAND, JUL 25-29, 2004},
title = {Development of radiation tolerant semiconductor detectors for the Super-LHC},
year = {2005},
journal = {Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment},
pages = {99--107},
keywords = {radiation , semiconductor detectors , SuperLHC , Therapeutics. Pharmacology, Instrumentation, Nuclear and High Energy Physics},
url = {http://strathprints.strath.ac.uk/37547/},
abstract = {The envisaged upgrade of the Large Hadron Collider (LHC) at CERN towards the Super-LHC (SLHC) with a 10 times increased luminosity of 1035 cm?2 s?1 will present severe challenges for the tracking detectors of the SLHC experiments. Unprecedented high radiation levels and track densities and a reduced bunch crossing time in the order of 10 ns as well as the need for cost effective detectors have called for an intensive R\&D program. The CERN RD50 collaboration ?Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders? is working on the development of semiconductor sensors matching the requirements of the SLHC. Sensors based on defect engineered silicon like Czochralski, epitaxial and oxygen enriched silicon have been developed. With 3D, Semi-3D and thin detectors new detector concepts have been evaluated and a study on the use of standard and oxygen enriched p-type silicon detectors revealed a promising approach for radiation tolerant cost effective devices. These and other most recent advancements of the RD50 collaboration are presented.}
}

The proposed luminosity upgrade of the Large Hadron Collider (S-LHC) at CERN will demand the innermost layers of the vertex detectors to sustain fluences of about 1016 hadrons/cm2. Due to the high multiplicity of tracks, the required spatial resolution and the extremely harsh radiation field new detector concepts and semiconductor materials have to be explored for a possible solution of this challenge. The CERN RD50 collaboration ?Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders? has started in 2002 an R&D program for the development of detector technologies that will fulfill the requirements of the S-LHC. Different strategies are followed by RD50 to improve the radiation tolerance. These include the development of defect engineered silicon like Czochralski, epitaxial and oxygen-enriched silicon and of other semiconductor materials like SiC and GaN as well as extensive studies of the microscopic defects responsible for the degradation of irradiated sensors. Further, with 3D, Semi-3D and thin devices new detector concepts have been evaluated. These and other recent advancements of the RD50 collaboration are presented and discussed.

@article{strathprints37508,
volume = {552},
number = {1-2},
month = {October},
author = {E Fretwurst and J Adey and A Al-Ajili and G Alfieri and PP Allport and M Artuso and S Assouak and BS Avset and L Barabashi and A Barcz and R Bates and SF Biagi and GM Bilei and D Bisello and A Blue and A Blumenau and V Boisvert and G Bolla and G Bondarenko and E Borchi and L Borrello and D Bortoletto and M Boscardin and L Bosisio and TJV Bowcock and TJ Brodbeck and J Broz and M Bruzzi and A Brzozowski and M Buda and P Buhmann and C Buttar and F Campabadal and D Campbell and A Candelori and G Casse and A Cavallini and S Charron and A Chilingarov and D Chren and V Cindro and P Collins and R Coluccia and D Contarato and J Coutinho and D Creanza and L Cunningham and GF Dalla Betta and I Dawson and W de Boer and M De Palma and R Demina and P Dervan and S Dittongo and Z Dolezal and A Dolgolenko and T Eberlein and V Eremin and C Fall and F Fasolo and T Ferbel and F Fizzotti and C Fleta and E Focardi and E Forton and C Garcia and JE Garcia-Navarro and E Gaubas and MH Genest and KA Gill and K Giolo and M Glaser and C Goessling and V Golovine and SG Sevilla and I Gorelov and J Goss and AG Bates and G Gregoire and P Gregori and E Grigoriev and AA Grillo and A Groza and J Guskov and L Haddad and J Harkonen and F Hauler and M Hoeferkamp and F Honniger and T Horazdovsky and R Horisberger and M Horn and A Houdayer and Benjamin Hourahine and G Hughes and I Ilyashenko and K Irmscher and A Ivanov and K Jarasiunas and KMH Johansen and BK Jones and R Jones and C Joram and L Jungermann and E Kalinina and P Kaminski and A Karpenko and A Karpov and V Kazlauskiene and V Kazukauskas and V Khivrich and V Khomenkov and J Kierstead and J Klaiber-Lodewigs and R Klingenberg and P Kodys and Z Kohout and S Korjenevski and M Koski and R Kozlowski and M Kozodaev and G Kramberger and O Krasel and A Kuznetsov and S Kwan and S Lagomarsino and K Lassila-Perini and V Lastovetsky and G Latino and I Lazanu and S Lazanu and A Lebedev and C Lebel and K Leinonen and C Leroy and Z Li and G Lindstrom and V Linhart and P Litovchenko and A Litovchenko and AL Giudice and M Lozano and Z Luczynski and P Luukka and A Macchiolo and LF Makarenko and I Mandic and C Manfredotti and N Manna and SM Garcia and S Marunko and K Mathieson and J Melone and D Menichelli and A Messineo and J Metcalfe and S Miglio and M Mikuz and J Miyamoto and M Moll and E Monakhov and F Moscatelli and D Naoumov and E Nossarzewska-Orlowska and J Nysten and P Olivero and V Oshea and T Palviainen and C Paolini and C Parkes and D Pesseri and U Pein and G Pellegrini and L Perera and M Petasecca and C Plemonte and GU Pignatel and N Pinho and I Pintilie and L Pintilie and L Polivtsev and P Polozov and A Popa and J Popule and S Pospisil and A Pozza and V Radicci and JM Rafi and R Rando and R Roeder and T Rohe and S Ronchin and C Rott and A Roy and A Ruzin and HFW Sadrozinski and S Sakalauskas and M Scaringella and L Schiavulli and S Schnetzer and B Schumm and S Sciortino and A Scorzoni and G Segneri and S Seidel and A Seiden and G Sellberg and P Sellin and D Sentenac and I Shipsey and P Sicho and T Sloan and M Solar and S Son and B Sopko and V Sopko and N Spencer and J Stahl and D Stolze and R Stone and J Storasta and N Strokan and M Sudzius and B Surma and A Suvorov and BG Svensson and P Tipton and M Tomasek and A Tsvetkov and E Tuominen and E Tuovinen and T Tuuva and M Tylchin and H Uebersee and J Uher and M Ullan and JV Vaitkus and J Velthuis and E Verbitskaya and V Vrba and G Wagner and I Wilhelm and S Worm and V Wright and R Wunstorf and Y Yiuri and P Zabierowski and A Zaluzhny and M Zavrtanik and M Zen and V Zhukov and N Zorzi and Benjamin Hourahine},
note = {5th International Conference on Radiation Effects on Semiconductor Materials, Detectors and Devices, Florence, ITALY, OCT 10-13, 2004},
title = {Recent advancements in the development of radiation hard semiconductor detectors for S-LHC},
year = {2005},
journal = {Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment},
pages = {7--19},
keywords = {optics , photonics, semiconductor detectors , radiation, Optics. Light, Instrumentation, Nuclear and High Energy Physics},
url = {http://strathprints.strath.ac.uk/37508/},
abstract = {The proposed luminosity upgrade of the Large Hadron Collider (S-LHC) at CERN will demand the innermost layers of the vertex detectors to sustain fluences of about 1016 hadrons/cm2. Due to the high multiplicity of tracks, the required spatial resolution and the extremely harsh radiation field new detector concepts and semiconductor materials have to be explored for a possible solution of this challenge. The CERN RD50 collaboration ?Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders? has started in 2002 an R\&D program for the development of detector technologies that will fulfill the requirements of the S-LHC. Different strategies are followed by RD50 to improve the radiation tolerance. These include the development of defect engineered silicon like Czochralski, epitaxial and oxygen-enriched silicon and of other semiconductor materials like SiC and GaN as well as extensive studies of the microscopic defects responsible for the degradation of irradiated sensors. Further, with 3D, Semi-3D and thin devices new detector concepts have been evaluated. These and other recent advancements of the RD50 collaboration are presented and discussed.}
}

An option of increasing the luminosity of the Large Hadron Collider (LHC) at CERN to 1035 cm?2 s?1 has been envisaged to extend the physics reach of the machine. An efficient tracking down to a few centimetres from the interaction point will be required to exploit the physics potential of the upgraded LHC. As a consequence, the semiconductor detectors close to the interaction region will receive severe doses of fast hadron irradiation and the inner tracker detectors will need to survive fast hadron fluences of up to above 1016 cm?2. The CERN-RD50 project ?Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders? has been established in 2002 to explore detector materials and technologies that will allow to operate devices up to, or beyond, this limit. The strategies followed by RD50 to enhance the radiation tolerance include the development of new or defect engineered detector materials (SiC, GaN, Czochralski and epitaxial silicon, oxygen enriched Float Zone silicon), the improvement of present detector designs and the understanding of the microscopic defects causing the degradation of the irradiated detectors. The latest advancements within the RD50 collaboration on radiation hard semiconductor detectors will be reviewed and discussed in this work.

@article{strathprints37499,
volume = {541},
number = {1-2},
month = {April},
author = {M Bruzzi and J Adey and A Al-Ajili and P Alexandrov and G Alfieri and PP Allport and A Andreazza and M Artuso and S Assouak and BS Avset and L Barabash and E Baranova and A Barcz and A Basile and R Bates and N Belova and SF Biagi and GM Bilei and D Bisello and A Blue and B Blumenau and V Boisvert and G Bolla and G Bondarenko and E Borchi and L Borrello and D Bortoletto and M Boscardin and L Bosisio and TJV Bowcock and TJ Brodbeck and J Broz and A Brukhanov and A Brzozowski and M Buda and P Buhmann and C Buttar and F Campabadal and D Campbell and A Candelori and G Casse and A Cavallini and A Chilingarov and D Chren and V Cindro and M Citterio and P Collins and R Coluccia and D Contarato and J Coutinho and D Creanza and W Cunningham and V Cvetkov and GF Dalla Betta and G Davies and I Dawson and W de Boer and M De Palma and R Demina and P Dervan and A Dierlamm and S Dittongo and L Dobrzanski and Z Dolezal and A Dolgolenko and T Eberlein and V Eremin and C Fall and F Fasolo and T Ferbel and F Fizzotti and C Fleta and E Focardi and E Forton and S Franchenko and E Fretwurst and F Gamaz and C Garcia and JE Garcia-Navarro and E Gaubas and MH Genest and KA Gill and K Giolo and M Glaser and C Goessling and V Golovine and SG Sevilla and I Gorelov and J Goss and A Gouldwell and G Gregoire and P Gregori and E Grigoriev and C Grigson and A Grillo and A Groza and J Guskov and L Haddad and J Harkonen and R Harding and F Hauler and S Hayama and M Hoeferkamp and F Honniger and T Horazdovsky and R Horisberger and M Horn and A Houdayer and Benjamin Hourahine and A Hruban and G Hughes and I Ilyashenko and K Irmscher and A Ivanov and K Jarasiunas and T Jin and BK Jones and R Jones and C Joram and L Jungermann and E Kalinina and P Kaminski and A Karpenko and A Karpov and V Kazlauskiene and V Kazukauskas and V Khivrich and V Khomenkov and J Kierstead and J Klaiber-Lodewigs and M Kleverman and R Klingenberg and P Kodys and Z Kohout and S Korjenevski and A Kowalik and R Kozlowski and M Kozodaev and G Kramberger and O Krasel and A Kuznetsov and S Kwan and S Lagomarsino and T Lari and K Lassila-Perini and V Lastovetsky and G Latino and S Latushkin and S Lazanu and I Lazanu and C Lebel and K Leinonen and C Leroy and Z Li and G Lindstrom and L Lindstrom and V Linhart and A Litovchenko and P Litovchenko and V Litvinov and A Lo Giudice and M Lozano and Z Luczynski and P Luukka and A Macchiolo and A Mainwood and LF Makarenko and I Mandic and C Manfredotti and SM Garcia and S Marunko and K Mathieson and A Mozzanti and J Melone and D Menichelli and C Meroni and A Messineo and S Miglio and M Mikuz and J Miyamoto and M Moll and E Monakhov and F Moscatelli and L Murin and F Nava and D Naoumov and E Nossarzewska-Orlowska and S Nummela and J Nysten and P Olivero and V Oshea and T Palviainen and C Paolini and C Parkes and D Passeri and U Pein and G Pellegrini and L Perera and M Petasecca and B Piatkowski and C Piemonte and GU Pignatel and N Pinho and I Pintilie and L Pintilie and L Polivtsev and P Polozov and AI Popa and J Popule and S Pospisil and G Pucker and V Radicci and JM Rafi and F Ragusa and M Rahman and R Rando and R Roeder and T Rohe and S Ronchin and C Rott and P Roy and A Roy and A Ruzin and A Ryazanov and HFW Sadrozinski and S Sakalauskas and M Scaringella and L Schiavulli and S Schnetzer and B Schumm and S Sciortino and A Scorzoni and G Segneri and S Seidel and A Seiden and G Sellberg and P Sellin and D Sentenac and I Shipsey and P Sicho and T Sloan and M Solar and S Son and B Sopko and N Spencer and J Stahl and I Stavitski and D Stolze and R Stone and J Storasta and N Strokan and W Strupinski and M Sudzius and B Surma and J Suuronen and A Suvorov and BG Svensson and P Tipton and M Tomasek and C Troncon and A Tsvetkov and E Tuominen and E Tuovinen and T Tuuva and M Tylchin and H Uebersee and J Uher and M Ullan and JV Vaitkus and P Vanni and J Velthuis and G Verzellesi and E Verbitskaya and V Vrba and G Wagner and I Wilhelm and S Worm and V Wright and R Wunstorf and P Zablerowski and A Zaluzhny and M Zavrtanik and M Zen and V Zhukov and N Zorzi and Benjamin Hourahine},
note = {5th International Symposium on Development and Application of Semiconductor Tracking Detectors, Hiroshima, JAPAN, JUN 14-17, 2004},
title = {Radiation-hard semiconductor detectors for SuperLHC},
year = {2005},
journal = {Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment},
pages = {189--201},
keywords = {semiconductor detectors , SuperLHC , radiation, Therapeutics. Pharmacology, Instrumentation, Nuclear and High Energy Physics},
url = {http://strathprints.strath.ac.uk/37499/},
abstract = {An option of increasing the luminosity of the Large Hadron Collider (LHC) at CERN to 1035 cm?2 s?1 has been envisaged to extend the physics reach of the machine. An efficient tracking down to a few centimetres from the interaction point will be required to exploit the physics potential of the upgraded LHC. As a consequence, the semiconductor detectors close to the interaction region will receive severe doses of fast hadron irradiation and the inner tracker detectors will need to survive fast hadron fluences of up to above 1016 cm?2. The CERN-RD50 project ?Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders? has been established in 2002 to explore detector materials and technologies that will allow to operate devices up to, or beyond, this limit. The strategies followed by RD50 to enhance the radiation tolerance include the development of new or defect engineered detector materials (SiC, GaN, Czochralski and epitaxial silicon, oxygen enriched Float Zone silicon), the improvement of present detector designs and the understanding of the microscopic defects causing the degradation of the irradiated detectors. The latest advancements within the RD50 collaboration on radiation hard semiconductor detectors will be reviewed and discussed in this work.}
}

Study of the relationship between the composition and optical energies of InxGa1-xN has generated much interest and intrigue over the last decade and beyond. In this paper we describe data from InxGa1-xN epilayers covering the full range of composition (0 {\ensuremath{<}} x {\ensuremath{<}} 1), grown by both Metal-Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). In particular we concentrate on a set of state-of-the-art InN rich MBE layers (0.6 {\ensuremath{<}} x {\ensuremath{<}} 1.0). Wavelength dispersive X-ray microanalysis is employed for accurate measurement of the InN fraction and of the group III : group V ratio. The InN rich layers are shown to be highly stoichiometric. The composition results are correlated with luminescence spectra, which show peaks covering the range 1.3 to 0.7 eV. Inclusion of our data from sets of MOVPE and MBE epilayers with InN fractions up to 0.4, measured using identical techniques, allows the composition dependence of the luminescence peak energy to be plotted across the entire composition range. A quadratic fit gives good agreement with both the low-InN MOVPE and high-InN MBE samples but not for the intermediate region. Possible reasons for this are discussed.

@incollection{strathprints36886,
month = {July},
author = {Robert Martin and P.R. Edwards and S. Hernandez and K Wang and I Fernandez-Torrente and M. Kurouchi and Y. Nanishi and K.P. O'Donnell},
series = {Materials research society symposium proceedings},
booktitle = {GaN, AIN, InN and their alloys},
editor = {B. Gil and M. Kuzuhara and M. Manfra and Christian Wetzel},
title = {The composition dependence of the optical properties of InN-rich InGaN grown by MBE},
address = {Warrendale},
publisher = {Materials Research Society},
year = {2005},
pages = {119--124},
keywords = {band-gap, alloys , Physics},
url = {http://strathprints.strath.ac.uk/36886/},
abstract = {Study of the relationship between the composition and optical energies of InxGa1-xN has generated much interest and intrigue over the last decade and beyond. In this paper we describe data from InxGa1-xN epilayers covering the full range of composition (0 {\ensuremath{<}} x {\ensuremath{<}} 1), grown by both Metal-Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). In particular we concentrate on a set of state-of-the-art InN rich MBE layers (0.6 {\ensuremath{<}} x {\ensuremath{<}} 1.0). Wavelength dispersive X-ray microanalysis is employed for accurate measurement of the InN fraction and of the group III : group V ratio. The InN rich layers are shown to be highly stoichiometric. The composition results are correlated with luminescence spectra, which show peaks covering the range 1.3 to 0.7 eV. Inclusion of our data from sets of MOVPE and MBE epilayers with InN fractions up to 0.4, measured using identical techniques, allows the composition dependence of the luminescence peak energy to be plotted across the entire composition range. A quadratic fit gives good agreement with both the low-InN MOVPE and high-InN MBE samples but not for the intermediate region. Possible reasons for this are discussed.}
}

The behavior of the E-2 and A(1)(LO) optical phonons in InxGa1-xN has been analyzed by Raman scattering over the whole composition range. The frequencies of the E-2 and A(1)(LO) modes decrease with increasing InN fraction. These modes display a significant broadening for an InN fraction of approximate to 60\% and their linewidth decreases towards both ends of the composition range as a consequence of reduced cation disorder. Our results show a one-mode behavior for, both E-2 and A(1)(LO) modes of InGaN.

The following is a report on the Evening Rump Session on InN held as part of the 2004 International Workshop on Nitride Semiconductors. It summarises (1) the presentations given by the 5 panellists covering data generated from theory and a wide range of experimental techniques relating to the properties of InN, in particular its bandgap and (2) the subsequent discussion. The most recent parameter-free electronic-structure calculations predict a value for the InN bandgap of 0.8 +/- 0.4 eV; experimental results obtained from a wide range of InN samples point to a bandgap around 0.7 eV, or to a bandgap around 1.3 eV. The interpretation of available data is hotly contested, not surprisingly a definitive conclusion on the true value of the bandgap of InN was not reached during the Rump Session. It was agreed that InN is a difficult material to grow and its properties vary depending on how and where it is grown. However with mobilities of order 4000 cm(2)/Vs, high saturation velocities and the absorption wavelengths of InGaN spanning the visible, InN is a material with huge potential.

@InCollection{strathprints32459,
author = {Carol Trager-Cowan},
title = {Report on the evening rump session on InN - July 21, 2004 at the 2004 international workshop on nitride semiconductors},
booktitle = {Physica status solidi C - conferences and critical reviews},
publisher = {Wiley-VCH},
year = {2005},
editor = {M Stutzmann},
volume = {2},
series = {Physica status solidi c - current topics in solid state physics},
pages = {2240--2245},
address = {Weinheim},
abstract = {The following is a report on the Evening Rump Session on InN held as part of the 2004 International Workshop on Nitride Semiconductors. It summarises (1) the presentations given by the 5 panellists covering data generated from theory and a wide range of experimental techniques relating to the properties of InN, in particular its bandgap and (2) the subsequent discussion. The most recent parameter-free electronic-structure calculations predict a value for the InN bandgap of 0.8 +/- 0.4 eV; experimental results obtained from a wide range of InN samples point to a bandgap around 0.7 eV, or to a bandgap around 1.3 eV. The interpretation of available data is hotly contested, not surprisingly a definitive conclusion on the true value of the bandgap of InN was not reached during the Rump Session. It was agreed that InN is a difficult material to grow and its properties vary depending on how and where it is grown. However with mobilities of order 4000 cm(2)/Vs, high saturation velocities and the absorption wavelengths of InGaN spanning the visible, InN is a material with huge potential.},
keywords = {molecular-beam epitaxy, optical-properties, energy-gap, alloys, growth, absorption, films, Physics},
url = {http://strathprints.strath.ac.uk/32459/}
}

Superconducting thin films of Ndl-xCaxBa2Cu3O7-delta (x = 0.03 and 0.08) have been grown on single crystal SrTiO3 substrates by pulsed laser deposition. The statistical methods of Experimental Design and regression analysis were used to optimize the film properties and to understand the correlation between the growth parameters and film properties. The orientation of the films was investigated by x-ray diffraction. The surface morphology of the films was examined by atomic force microscopy and scanning tunnelling microscopy. Qualitative and quantitative elemental analyses of the films were carried out using electron probe microanalysis. Micro-Raman spectroscopy was used to study the oxygen sublattice vibrations of the films. The effect of annealing oil the superconducting transition temperature of the patterned films was also studied.

@article{strathprints31061,
volume = {38},
number = {1},
month = {January},
author = {R Palai and EJ Romans and RW Martin and FT Docherty and P Maas and CM Pegrum},
title = {Studies of growth, microstructure, IMP Raman spectroscopy and annealing effect of pulsed laser deposited Ca-doped NBCO thin films},
journal = {Journal of Physics D: Applied Physics},
pages = {51--61},
year = {2005},
keywords = {thin films , microstructure, raman spectroscopy , annealing effect , Ca-doped NBCO , microscopy, Optics. Light, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31061/},
abstract = {Superconducting thin films of Ndl-xCaxBa2Cu3O7-delta (x = 0.03 and 0.08) have been grown on single crystal SrTiO3 substrates by pulsed laser deposition. The statistical methods of Experimental Design and regression analysis were used to optimize the film properties and to understand the correlation between the growth parameters and film properties. The orientation of the films was investigated by x-ray diffraction. The surface morphology of the films was examined by atomic force microscopy and scanning tunnelling microscopy. Qualitative and quantitative elemental analyses of the films were carried out using electron probe microanalysis. Micro-Raman spectroscopy was used to study the oxygen sublattice vibrations of the films. The effect of annealing oil the superconducting transition temperature of the patterned films was also studied.}
}

Three series of 1 mu m thick Ga1-xMnxAs films with different Mn composition have been characterized using high-resolution x-ray diffraction (HRXRD). The results show that they are highly-crystalline and the growth is reproducible. The Mn compositions have also been measured by other popular methods: in situ ion gauge, x-ray fluorescence (XRF), electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS). The results show that the Mn concentrations measured by different methods are different and the difference between them becomes smaller with increasing Mn content.

@article{strathprints31060,
volume = {20},
number = {5},
month = {May},
author = {LX Zhao and RP Campion and PF Fewster and RW Martin and BY Ber and AP Kovarsky and CR Staddon and KY Wang and KW Edmonds and CT Foxon and BL Gallagher},
title = {Determination of the Mn concentration in GaMnAs},
journal = {Semiconductor Science and Technology},
pages = {369--373},
year = {2005},
keywords = {GaMnAs, semiconductors, x-ray diffraction, Optics. Light, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31060/},
abstract = {Three series of 1 mu m thick Ga1-xMnxAs films with different Mn composition have been characterized using high-resolution x-ray diffraction (HRXRD). The results show that they are highly-crystalline and the growth is reproducible. The Mn compositions have also been measured by other popular methods: in situ ion gauge, x-ray fluorescence (XRF), electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS). The results show that the Mn concentrations measured by different methods are different and the difference between them becomes smaller with increasing Mn content.}
}

Photoluminescence (PL) of Eu-implanted GaN epilayers grown by Metalorganic Vapour Phase Epitaxy (MOVPE) was studied as a function of temperature. The implantation was done at ion energies of 75 keV, 200 keV and 350 keV with doses of 10(14) CM-2 and 10(15) CM-2. PL spectra of all samples show the emission 5 7 31 line assigned to the D-5(0)-F-7(2) transition of EU3+ in GaN to be split into three spectral components at 620.7 nm, 621.6 nm and 622.5 nm. The split lines are seen to have very different temperature dependences of integrated intensity. Such splitting might be explained by EU3+ ion site multiplicity. The variation of the temperature quenching factor of the PL integrated intensity from sample to sample and from line to line suggests that optically active Eu3+ ions are coupled to defects and impurities, thus forming complexes with different energy position of the carrier trapping level in the bandgap of GaN. The appearance and quenching of an additional PL line at 617.3 nm with increasing temperature is observed in the range of 13-295 K.

@article{strathprints31059,
volume = {242},
number = {7},
month = {June},
author = {V Katchkanov and KP O'Donnell and S Dalmasso and RW Martin and A Braud and Y Nakanishi and A Wakahara and A Yoshida},
title = {Photoluminescence studies of Eu-implanted GaN epilayers},
journal = {Physica Status Solidi B},
pages = {1491--1496},
year = {2005},
keywords = {photoluminescence, luminescence, vapour phase epitaxy, Optics. Light, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31059/},
abstract = {Photoluminescence (PL) of Eu-implanted GaN epilayers grown by Metalorganic Vapour Phase Epitaxy (MOVPE) was studied as a function of temperature. The implantation was done at ion energies of 75 keV, 200 keV and 350 keV with doses of 10(14) CM-2 and 10(15) CM-2. PL spectra of all samples show the emission 5 7 31 line assigned to the D-5(0)-F-7(2) transition of EU3+ in GaN to be split into three spectral components at 620.7 nm, 621.6 nm and 622.5 nm. The split lines are seen to have very different temperature dependences of integrated intensity. Such splitting might be explained by EU3+ ion site multiplicity. The variation of the temperature quenching factor of the PL integrated intensity from sample to sample and from line to line suggests that optically active Eu3+ ions are coupled to defects and impurities, thus forming complexes with different energy position of the carrier trapping level in the bandgap of GaN. The appearance and quenching of an additional PL line at 617.3 nm with increasing temperature is observed in the range of 13-295 K.}
}

Superconducting thin films of Nd1-xBa2+xCu3O7-delta have been grown on single crystal SrTiO3 substrates by pulsed laser deposition (PLD) using an off-stoichiometric Nd0.97Ba2.03Cu3O6 (Ba-rich NBCO) target. The statistical methods of Experimental design and regression analysis were used to understand and optimise the growth mechanism. The structural properties of both the target and the films were investigated by X-ray diffraction (XRD). The surface morphology of the films was examined by atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). Electron probe microanalysis (EPMA) using a scanning electron microscope equipped with wavelength-dispersive X-ray (WDX) spectrometers was used to carry out qualitative and quantitative analysis of both the target and films. Micro-Raman spectroscopy was used to study the oxygen sublattice vibrations of both the target and the films. (c) 2005 Elsevier B.V. All rights reserved.

@article{strathprints31058,
volume = {424},
number = {1-2},
month = {August},
author = {R Palai and EJ Romans and RW Martin and FT Docherty and CM Pegrum},
title = {Growth mechanism, microstructure, EPMA and Raman studies of pulsed laser deposited Nd1-xBa2+xCu3O7-delta thin films},
journal = {Physica C: Superconductivity and its Applications},
pages = {57--71},
year = {2005},
keywords = {raman studies, microstructures, lasers, Optics. Light, Energy Engineering and Power Technology, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/31058/},
abstract = {Superconducting thin films of Nd1-xBa2+xCu3O7-delta have been grown on single crystal SrTiO3 substrates by pulsed laser deposition (PLD) using an off-stoichiometric Nd0.97Ba2.03Cu3O6 (Ba-rich NBCO) target. The statistical methods of Experimental design and regression analysis were used to understand and optimise the growth mechanism. The structural properties of both the target and the films were investigated by X-ray diffraction (XRD). The surface morphology of the films was examined by atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). Electron probe microanalysis (EPMA) using a scanning electron microscope equipped with wavelength-dispersive X-ray (WDX) spectrometers was used to carry out qualitative and quantitative analysis of both the target and films. Micro-Raman spectroscopy was used to study the oxygen sublattice vibrations of both the target and the films. (c) 2005 Elsevier B.V. All rights reserved.}
}

Hyperspectral mapping is a very powerful technique for determining the cathodoluminescence (CL) emission characteristics and chemical compositions of materials over relatively large areas and at micron to sub-micron spatial resolutions (Lee et al. 2005). Using a Cameca SX100 electron probe at Strathclyde University, which is equipped with a silicon CCD spectrograph and energy- and wavelengthdispersive X-ray detectors, we have investigated spatial and spectral variations in the CL properties of meteoric calcite cements from the Upper Permian of the UK and perthitic alkali feldspars (Na- and K-rich intergrowths) from the Precambrian Klokken syenite, Greenland.

A facility has been developed to acquire hyperspectral cathodoluminescence (CL) images simultaneously with X-ray composition data. Based around an electron microprobe, the system uses a built-in Cassegrain microscope to efficiently couple emitted light directly into the entrance slit of an optical spectrograph. A cooled array detector allows the parallel acquisition of CL spectra, which are then built up into a multidimensional data-cube containing the full set of spectrally- and spatially-resolved information for later analysis. This setup has the advantage of allowing wavelength-dispersive X-ray (WDX) data to be recorded concurrently, providing a powerful technique for the direct comparison of luminescent and compositional properties of materials. The combination of beam and sample scanning thus allows the correlation of composition and luminescence inhomogeneities on length scales ranging from a few cm to sub-micron.

@article{strathprints31056,
volume = {69},
number = {10, Su},
month = {May},
author = {PR Edwards and RW Martin and MR Lee},
note = {15th Annual V M Goldschmidt Conference, Moscow, ID, MAY, 2005},
title = {Simultaneous cathodoluminescence hyperspectral imaging and X-ray microanalysis},
year = {2005},
journal = {Geochimica et Cosmochimica Acta},
pages = {A591},
keywords = {hyperspectral imaging, X-ray microanalysis , cathodoluminescence , Optics. Light, Geochemistry and Petrology},
url = {http://strathprints.strath.ac.uk/31056/},
abstract = {A facility has been developed to acquire hyperspectral cathodoluminescence (CL) images simultaneously with X-ray composition data. Based around an electron microprobe, the system uses a built-in Cassegrain microscope to efficiently couple emitted light directly into the entrance slit of an optical spectrograph. A cooled array detector allows the parallel acquisition of CL spectra, which are then built up into a multidimensional data-cube containing the full set of spectrally- and spatially-resolved information for later analysis. This setup has the advantage of allowing wavelength-dispersive X-ray (WDX) data to be recorded concurrently, providing a powerful technique for the direct comparison of luminescent and compositional properties of materials. The combination of beam and sample scanning thus allows the correlation of composition and luminescence inhomogeneities on length scales ranging from a few cm to sub-micron.}
}

The aim of this work is to study the dependence of the structural properties of Cu(In,Ga)Se-2 polycrystalline thin films prepared by a two-step selenization of co-evaporated metallic precursors in Se-containing environment under N-2 gas flow. Characterizations included studies of morphological features, formation of crystalline phases and the depth compositional uniformity of the final thin films. From these studies optimum growth parameters were determined for the preparation of: high-quality chalcopyrite thin films with structural and compositional properties suitable for solar cell applications. The formation and optoelectronic properties of glass/Mo/Cu(In,Ga)Se-2/CdS/ZnO/Al-Ni thin film solar cells are also reported. The better conversion efficiencies were around 8.0 (C) 2005 Elsevier B.V. All rights reserved.

Imaging of cathodoluminescence (CL) emission from carbonate minerals by scanning electron microscopy (SEM) is problematic owing to the slow rate at which CL decays from each point as the electron beam is scanned over an area (the phenomenon of phosphorescence). Using SEM-CL and a newly developed electron probe-based technique of hyperspectral mapping, we have evaluated methods that have been proposed to overcome phosphorescence. With the dwell-time technique, the duration of time that the electron beam is static is increased such that CL emission from a given point makes a negligible contribution to the total signal from subsequent points. The dwell-time required to form sharp SEM-CL images of calcite that has a high intensity of luminescence at orange wavelengths is equal to or greater than 6.4 milliseconds, whereas calcite that luminesces predominantly at ultraviolet to blue wavelengths can be imaged using submillisecond dwell times. By using longer dwell times ({\ensuremath{>}} 1000 ms), CL and X-ray spectra can also be acquired from each point to form hyperspectral maps. The limited-wavelength imaging technique employs optical filters to excise slowly decaying long-wavelength emission so that the image is formed only using the more rapidly decaying ultraviolet to blue wavelengths, allowing shorter dwell times to be used. Both techniques have some disadvantages. The main drawback of the dwell-time technique is the long period of time required to acquire high-resolution images whereas the success of limited-wavelength imaging may depend on the sensitivity of the CL detector being used. CL images and emission spectra acquired by hyperspectral mapping also show that there is a nonlinear relationship between luminescence intensity variations at ultraviolet to blue wavelengths and intensity variations at orange wavelengths, indicating that short-wavelength emission is an imperfect proxy for zoning at longer wavelengths. Additionally, we have been unable to identify the controls on spatial variations in the intensity of ultraviolet to blue luminescence, although our data discount Fe2+ concentrations as being the sole determinant. By using hyperspectral mapping in combination with electron-probe microanalysis to obtain CL and X-ray spectra from the same micrometer-sized volume of a material, it is now possible to understand and quantify the controls on activation and quenching of CL in calcite and other minerals such as apatite and zircon.

@article{strathprints30536,
volume = {75},
number = {2},
month = {March},
author = {M R Lee and R W Martin and C Trager-Cowan and P R Edwards},
title = {Imaging of cathodoluminescence zoning in calcite by scanning electron microscopy and hyperspectral mapping},
journal = {Journal of Sedimentary Research Section A: Sedimentary Petrology and Processes},
pages = {313--322},
year = {2005},
keywords = {time-resolved luminescence, minerals, carbonate, dolomite, Physics, Stratigraphy, Geology},
url = {http://strathprints.strath.ac.uk/30536/},
abstract = {Imaging of cathodoluminescence (CL) emission from carbonate minerals by scanning electron microscopy (SEM) is problematic owing to the slow rate at which CL decays from each point as the electron beam is scanned over an area (the phenomenon of phosphorescence). Using SEM-CL and a newly developed electron probe-based technique of hyperspectral mapping, we have evaluated methods that have been proposed to overcome phosphorescence. With the dwell-time technique, the duration of time that the electron beam is static is increased such that CL emission from a given point makes a negligible contribution to the total signal from subsequent points. The dwell-time required to form sharp SEM-CL images of calcite that has a high intensity of luminescence at orange wavelengths is equal to or greater than 6.4 milliseconds, whereas calcite that luminesces predominantly at ultraviolet to blue wavelengths can be imaged using submillisecond dwell times. By using longer dwell times ({\ensuremath{>}} 1000 ms), CL and X-ray spectra can also be acquired from each point to form hyperspectral maps. The limited-wavelength imaging technique employs optical filters to excise slowly decaying long-wavelength emission so that the image is formed only using the more rapidly decaying ultraviolet to blue wavelengths, allowing shorter dwell times to be used. Both techniques have some disadvantages. The main drawback of the dwell-time technique is the long period of time required to acquire high-resolution images whereas the success of limited-wavelength imaging may depend on the sensitivity of the CL detector being used. CL images and emission spectra acquired by hyperspectral mapping also show that there is a nonlinear relationship between luminescence intensity variations at ultraviolet to blue wavelengths and intensity variations at orange wavelengths, indicating that short-wavelength emission is an imperfect proxy for zoning at longer wavelengths. Additionally, we have been unable to identify the controls on spatial variations in the intensity of ultraviolet to blue luminescence, although our data discount Fe2+ concentrations as being the sole determinant. By using hyperspectral mapping in combination with electron-probe microanalysis to obtain CL and X-ray spectra from the same micrometer-sized volume of a material, it is now possible to understand and quantify the controls on activation and quenching of CL in calcite and other minerals such as apatite and zircon.}
}

Europium was implanted into GaN through a 10 nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300?C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300?C.

@article{strathprints3006,
volume = {85},
number = {14},
month = {October},
author = {K. Lorenz and U. Wahl and E. Alves and S. Dalmasso and R.W. Martin and K.P. O'Donnell and S. Ruffenach and O. Briot},
title = {High temperature annealing and optical activation of Eu implanted GaN},
journal = {Applied Physics Letters},
pages = {2712--2714},
year = {2005},
keywords = {high temperature annealing, optical activation, Eu implanted GaN, nanoscience, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/3006/},
abstract = {Europium was implanted into GaN through a 10 nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300?C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300?C.}
}

The intensity of Eu-related luminescence from ion-implanted GaN with a 10 nm thick AlN cap, both grown epitaxially by metal organic chemical vapor deposition (MOCVD) is increased markedly by high-temperature annealing at 1300 ?C. Photoluminescence (PL) and PL excitation (PLE) studies reveal a variety of Eu centers with different excitation mechanisms. High-resolution PL spectra at low temperature clearly show that emission lines ascribed to 5D0-7F2 ({\texttt{\char126}}622 nm), 5D0-7F3 ({\texttt{\char126}}664 nm), and 5D0-7F1 ({\texttt{\char126}}602 nm) transitions each consist of several peaks. PL excitation spectra of the spectrally resolved components of the 5D0-7F2 multiplet contain contributions from above-bandedge absorption by the GaN host, a GaN exciton absorption at 356 nm, and a broad subedge absorption band centred at {\texttt{\char126}}385 nm. Marked differences in the shape of the 5D0-7F2 PL multiplet are demonstrated by selective excitation via the continuum/exciton states and the below gap absorption band. The four strongest lines of the multiplet are shown to consist of two pairs due to different Eu3+ centers with different excitation mechanisms.

Local density functional calculations are carried out on Er, Eu, and Tm rare-earth (RE) dopants in hexagonal AlN. We find that the isolated impurities prefer to substitute for Al and, in contrast with isolated RE dopants in GaAs and GaN, REAl defects are electrically active and introduce deep donor levels around Ev+0.5 eV. RE complexes with oxygen and vacancies are discussed; some of these have deep levels in the upper third of the gap and could account for a threshold excitation energy around 4 eV observed for intra-f transitions at 465 and 478 nm in AlN:Tm.

Single-crystalline germanium wafers exposed to hydrogen and/or deuterium plasma are studied by means of Raman scattering. The Raman frequencies are compared to results of ab initio calculations. For samples treated with pure hydrogen, Raman measurements performed at a temperature of 80 K reveal two sharp lines at 3826 and 3834 cm?1 with an intensity ratio of 3:1, which are assigned to ortho- and para-H2 trapped at the interstitial T site of the lattice.

Magnesium doping is critically important in GaN device technology, since it provides the only viable method of producing layers with p-type conductivity. Electron probe microanalysis with wavelength dispersive x-ray spectrometry (WDX-EPMA) was used to measure magnesium atom concentrations in doped GaN films grown by metal organic vapour phase epitaxy (MOVPE). Our study compared the behaviour of a widely used magnesium source in MOVPE, bis(cyclopentadienyl) magnesium, when vaporized as a solid and as a proprietary two-phase source, Solution Cp2Mg?. The WDX-EPMA technique was capable of measuring [Mg] values in GaN layers at practically useful concentrations of 1019 cm?3 upwards. Excellent agreement in [Mg] values was obtained between [Mg] values measured by WDX-EPMA and the more widely used technique of secondary ion mass spectrometry (SIMS). A set of 12 GaN:Mg samples was studied by WDX-EPMA to investigate the dependence of [Mg] on the flow rate of the magnesium source into the MOVPE reactor, with other conditions held constant, including a growth set-point temperature of 1130 ?C. These measurements suggested a solid solubility limit at {\texttt{\char126}}1020 cm?3, consistent with previous studies. Up to a value of about half the saturation limit, [Mg] values were proportional to the magnesium source flow, and indicated magnesium atom incorporation from the gas phase with {\texttt{\char126}}11\% of the efficiency of gallium atoms. No systematic differences were seen between the behaviour of solid magnesocene and Solution Cp2Mg?. A more limited study of the temperature dependence of magnesium incorporation showed a reduction in incorporation of {\texttt{\char126}}40\% as the growth temperature was reduced from 1130 to 1090 ?C, consistent with kinetic control. Selected GaN:Mg samples were studied by Hall measurements and high-resolution x-ray diffraction. This work showed no systematic structural degradation of GaN:Mg close to the magnesium solubility limit. Our most conductive sample had a hole concentration of 4.4 {$\times$} 1017 cm?3, consistent with the expected generation of acceptors from only a small fraction of the magnesium atoms. We also discuss the relative capabilities of SIMS and WDX-EPMA in the context of analysing GaN:Mg samples. SIMS offers superior depth profiling capability and detection limits, whilst WDX-EPMA offers superior spatial resolution, non-destructive analysis, plus simultaneous imaging and cathodoluminescence spectroscopy.

@article{strathprints10019,
volume = {21},
number = {9},
title = {Wavelength-dispersive x-ray microanalysis as a novel method for studying magnesium doping in gallium nitride epitaxial films},
author = {C.J. Deatcher and K. Bejtka and R.W. Martin and S. Romani and H. Kheyrandish and L.M. Smith and S.A. Rushworth and C. Liu and M.G. Cheong and I.M. Watson},
year = {2005},
pages = {1287--1295},
journal = {Semiconductor Science and Technology},
keywords = {semiconductor, surfaces, interfaces, thin films , chemical physics, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/10019/},
abstract = {Magnesium doping is critically important in GaN device technology, since it provides the only viable method of producing layers with p-type conductivity. Electron probe microanalysis with wavelength dispersive x-ray spectrometry (WDX-EPMA) was used to measure magnesium atom concentrations in doped GaN films grown by metal organic vapour phase epitaxy (MOVPE). Our study compared the behaviour of a widely used magnesium source in MOVPE, bis(cyclopentadienyl) magnesium, when vaporized as a solid and as a proprietary two-phase source, Solution Cp2Mg?. The WDX-EPMA technique was capable of measuring [Mg] values in GaN layers at practically useful concentrations of 1019 cm?3 upwards. Excellent agreement in [Mg] values was obtained between [Mg] values measured by WDX-EPMA and the more widely used technique of secondary ion mass spectrometry (SIMS). A set of 12 GaN:Mg samples was studied by WDX-EPMA to investigate the dependence of [Mg] on the flow rate of the magnesium source into the MOVPE reactor, with other conditions held constant, including a growth set-point temperature of 1130 ?C. These measurements suggested a solid solubility limit at {\texttt{\char126}}1020 cm?3, consistent with previous studies. Up to a value of about half the saturation limit, [Mg] values were proportional to the magnesium source flow, and indicated magnesium atom incorporation from the gas phase with {\texttt{\char126}}11\% of the efficiency of gallium atoms. No systematic differences were seen between the behaviour of solid magnesocene and Solution Cp2Mg?. A more limited study of the temperature dependence of magnesium incorporation showed a reduction in incorporation of {\texttt{\char126}}40\% as the growth temperature was reduced from 1130 to 1090 ?C, consistent with kinetic control. Selected GaN:Mg samples were studied by Hall measurements and high-resolution x-ray diffraction. This work showed no systematic structural degradation of GaN:Mg close to the magnesium solubility limit. Our most conductive sample had a hole concentration of 4.4 {$\times$} 1017 cm?3, consistent with the expected generation of acceptors from only a small fraction of the magnesium atoms. We also discuss the relative capabilities of SIMS and WDX-EPMA in the context of analysing GaN:Mg samples. SIMS offers superior depth profiling capability and detection limits, whilst WDX-EPMA offers superior spatial resolution, non-destructive analysis, plus simultaneous imaging and cathodoluminescence spectroscopy.}
}

InxGa1?xN quantum dots have been fabricated by the selective growth of GaN micro-pyramid arrays topped with InGaN/GaN quantum wells. The spatially- and spectrally-resolved luminescence properties of these structures were measured using low-temperature micro-photoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit linewidths down to 650 {\ensuremath{\mu}}eV (limited by the spectrometer resolution). We describe the broadening of the luminescence peak from a single dot as a function of temperature and excitation power.

@article{strathprints10018,
volume = {202},
number = {3},
month = {February},
author = {R.W. Martin and P.R. Edwards and R.A. Taylor and J.H. Rice and J.H. Na and J.W. Robinson and J.D. Smith and C. Liu and I.M. Watson},
title = {Luminescence properties of isolated InGaN/GaN quantum dots},
journal = {Physica Status Solidi A - Applications and Materials Science},
pages = {372--376},
year = {2005},
keywords = {physics, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/10018/},
abstract = {InxGa1?xN quantum dots have been fabricated by the selective growth of GaN micro-pyramid arrays topped with InGaN/GaN quantum wells. The spatially- and spectrally-resolved luminescence properties of these structures were measured using low-temperature micro-photoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit linewidths down to 650 {\ensuremath{\mu}}eV (limited by the spectrometer resolution). We describe the broadening of the luminescence peak from a single dot as a function of temperature and excitation power.}
}

We present Raman-scattering measurements on InxGa1?xN over the entire composition range of the alloy. The frequencies of the A1(LO) and E2 modes are reported and show a good agreement with the one-mode behavior dispersion predicted by the modified random-element isodisplacement model. The A1(LO) mode displays a high intensity relative to the E2 mode due to resonant enhancement. For above band-gap excitation, the A1(LO) peak displays frequency shifts as a function of the excitation energy due to selective excitation of regions with different In contents, and strong multiphonon scattering up to 3LO is observed in outgoing resonance conditions.

@article{strathprints10017,
volume = {98},
title = {Raman-scattering study of the InGaN alloy over the whole composition range},
author = {S. Hernandez and R. Cusco and D. Pastor and L. Artus and K.P. O'Donnell and R.W. Martin and I.M. Watson and Y. Nanishi and E. Calleja},
year = {2005},
pages = {013511--03515},
journal = {Journal of Applied Physics},
keywords = {indium compounds, phonon dispersion relations, phonon-phonon interactions, , spectral line shift, energy gap, Raman spectra, semiconductor epitaxial layers, gallium compounds, III-V semiconductors, wide band gap semiconductors , Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/10017/},
abstract = {We present Raman-scattering measurements on InxGa1?xN over the entire composition range of the alloy. The frequencies of the A1(LO) and E2 modes are reported and show a good agreement with the one-mode behavior dispersion predicted by the modified random-element isodisplacement model. The A1(LO) mode displays a high intensity relative to the E2 mode due to resonant enhancement. For above band-gap excitation, the A1(LO) peak displays frequency shifts as a function of the excitation energy due to selective excitation of regions with different In contents, and strong multiphonon scattering up to 3LO is observed in outgoing resonance conditions.}
}

AlInN alloys achieve an in-plane lattice match to hexagonal GaN at an indium nitride mole fraction of 18\%. Meanwhile Al0.82In0.18N displays a refractive index contrast of 7\% with GaN at visible wavelengths. We illustrate the use of Al0.82In0.18N insertion layers to control layer thicknesses during homoepitaxial growth of GaN-based microcavities, using in situ optical reflectometry. The structures discussed are 3 /2 microcavities incorporating distributed InGaN quantum wells tailored for emission at 400 nm. As-grown samples have been characterised by techniques including cathodoluminescence spectroscopy. In addition to their role in growth monitoring, there are several post-growth processing steps in which Al0.82In0.18N insertion layers can assist microcavity fabrication. We focus here on a demonstration of the 1:5 etch rate selectivity obtainable between Al0.82In0.18{\ensuremath{<}}/SUB {\ensuremath{>}}N and GaN in reactive ion etching

An array of pyramids containing templated InGaN/GaN quantum wells have been fabricated using selective overgrowth above patterned silica masks and studied using cathodoluminescence hyperspectral imaging. The cathodoluminescence reveals bright luminescence at the peaks of the micropyramids, red shifted from that due to similar, conventionally grown planar quantum wells. The possibility that this emission is due to quantum dots is discussed.

A novel technique based on the Fresnel diffraction effect for the fabrication of sub-micron ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an external diameter of 1.5 m and a wall width of 500 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 5 nm blue shift observed in the cathodoluminescence spectra can be attributed to band-filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated.

The application of cathodoluminescence spectral mapping to the characterisation of a range III-nitride semiconductor structures is described. Details are presented of the instrumentation developed to carry out such measurements using an electron probe micro-analyser. The spatial resolution of the luminescence data is {$\sim$}100 nm. The technique is enhanced by the ability to simultaneously perform X-ray microanalysis and electron imaging. Results are presented from epitaxially laterally overgrown GaN and InGaN/GaN structures using both single-layer SiO2 and multilayer SiO2/ZrO2 masks. Effects of strain and microcavity formation are resolved. Application of the technique to InGaN epilayers shows spatially-dependent shifts in the peak wavelength of the luminescence spectrum which correlate directly with microscopic variations in the indium content. Regions emitting at lower energy and with decreased intensity are shown to have higher InN contents, mirroring equivalent macroscopic observations. Finally the spectral mapping technique is used to analyse the luminescence from micron-scale selectively grown III-N pyramids, indicating possible formation of quantum dots at the sharp tips.

@article{strathprints38505,
volume = {201},
number = {4},
month = {March},
author = {R.W. Martin and P.R. Edwards and K.P. O'Donnell and M.D. Dawson and C.W. Jeon and C. Liu and G.R. Rice and I.M. Watson},
title = {Cathodoluminescence spectral mapping of III-nitride structures},
journal = {Physica Status Solidi A - Applications and Materials Science},
pages = {665--672},
year = {2004},
keywords = {cathodoluminescence, spectral mapping , iII-nitride structures, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/38505/},
abstract = {The application of cathodoluminescence spectral mapping to the characterisation of a range III-nitride semiconductor structures is described. Details are presented of the instrumentation developed to carry out such measurements using an electron probe micro-analyser. The spatial resolution of the luminescence data is {$\sim$}100 nm. The technique is enhanced by the ability to simultaneously perform X-ray microanalysis and electron imaging. Results are presented from epitaxially laterally overgrown GaN and InGaN/GaN structures using both single-layer SiO2 and multilayer SiO2/ZrO2 masks. Effects of strain and microcavity formation are resolved. Application of the technique to InGaN epilayers shows spatially-dependent shifts in the peak wavelength of the luminescence spectrum which correlate directly with microscopic variations in the indium content. Regions emitting at lower energy and with decreased intensity are shown to have higher InN contents, mirroring equivalent macroscopic observations. Finally the spectral mapping technique is used to analyse the luminescence from micron-scale selectively grown III-N pyramids, indicating possible formation of quantum dots at the sharp tips.}
}

The behavior of the E2 and A1(LO) optical phonons in Inx Ga1-x N has been analyzed by Raman scattering over the whole composition range. The frequencies of the E2 and A1(LO) modes decrease with increasing InN fraction. These modes display a significant broadening for an InN fraction of {$\approx$} 60\% and their linewidth decreases towards both ends of the composition range as a consequence of reduced cation disorder. Our results show a one-mode behavior for both E2 and A1(LO) modes of InGaN.

The roughness in GaN/InGaN thin ?lms and multilayers was studied with Rutherford backscattering (RBS).Quantitative data analysis, including the determination of the roughness parameters, was made through the application of models developed for speci?c kinds of roughness and/or intermixing. In a ?rst step, the assumptions made in the development of the models were tested, and their limits of validity were established. In all cases, the models are valid for relatively small aspect ratios of the interface or surface structures analysed, and the roughness parameters should follow a Gaussian distribution. Within their limits of application and validity, the models used in the RBS data analysis are general, and can be used in the study of any given system. The RBS results for GaN/InGaN thin ?lms were compared to atomic force microscopy (AFM) and scanning electron microscopy experiments. In the samples within the application range of the models, excellent agreement was found between the roughness determined by RBS and the surface roughness measured with AFM for thin ?lms. Finally, in GaN/InGaN multiple quantum wells, the roughness/intermixing was found to increase with both the well composition and the number of wells grown in the stack, due to a deterioration of the structural quality with the amount of strain incorporated in the structure.

@Article{strathprints37257,
author = {N.P. Barradas and E. Alves and de Sousa Pereira, S. M. and V.V. Shvartsman and A.L. Kholkin and E.M. Ferreira Pereira Lopes and K.P. O'Donnell and C. Liu and C.J. Deatcher and I.M. Watson and M. Mayer},
title = {Roughness in gaN/InGaN films and multilayers determined with Rutherford backscattering},
journal = {Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms},
year = {2004},
volume = {217},
number = {3},
pages = {479--497},
abstract = {The roughness in GaN/InGaN thin ?lms and multilayers was studied with Rutherford backscattering (RBS).Quantitative data analysis, including the determination of the roughness parameters, was made through the application of models developed for speci?c kinds of roughness and/or intermixing. In a ?rst step, the assumptions made in the development of the models were tested, and their limits of validity were established. In all cases, the models are valid for relatively small aspect ratios of the interface or surface structures analysed, and the roughness parameters should follow a Gaussian distribution. Within their limits of application and validity, the models used in the RBS data analysis are general, and can be used in the study of any given system. The RBS results for GaN/InGaN thin ?lms were compared to atomic force microscopy (AFM) and scanning electron microscopy experiments. In the samples within the application range of the models, excellent agreement was found between the roughness determined by RBS and the surface roughness measured with AFM for thin ?lms. Finally, in GaN/InGaN multiple quantum wells, the roughness/intermixing was found to increase with both the well composition and the number of wells grown in the stack, due to a deterioration of the structural quality with the amount of strain incorporated in the structure.},
keywords = {Rutherford backscattering, multilayers, thin ?lm, indium gallium nitride, gallium nitride , roughness, Optics. Light, Instrumentation, Nuclear and High Energy Physics},
url = {http://strathprints.strath.ac.uk/37257/}
}

Despite recent progress in the growth of InN-rich InxGa1-xN alloys, the composition dependence of the InGaN bandgap and the size of the InN gap remain uncertain. We apply a combination of techniques, Electron Probe Microanalysis (X-ray fluorescence spectroscopy (XRF) in wavelength-dispersive mode) and Cathodoluminescence (CL) spectroscopy, to the first of these problems. Our method measures in situ the composition and the luminescence spectrum of almost coincident volumes of sample, of size about one cubic micron. The combination of microcomposition mapping with CL spectrum imaging produces very large E-P(x) datasets. (E-P is the peak energy of the emission band.) We discover an unexplained systematic difference in the E-P(x) dependences of samples grown by Molecular Beam Epitaxy and Metalorganic Vapour Phase Epitaxy with similar ranges of x from near zero to similar to0.4. The linear relationship previously established between the bandgap energy E-B, measured by absorption spectroscopy, and E-P for MOVPE samples allows an extrapolation of the MOVPE E-B(x) data to x = 1, representing pure InN, which yields a predicted gap of 0.7(1) eV. This is likely to underestimate the true value.

@Article{strathprints30538,
author = {K P O'Donnell and I Fernandez-Torrente and P R Edwards and R W Martin},
title = {The composition dependence of the InₓGa₁₋ₓN bandgap},
journal = {Journal of Crystal Growth},
year = {2004},
volume = {269},
number = {1},
pages = {100--105},
month = {August},
abstract = {Despite recent progress in the growth of InN-rich InxGa1-xN alloys, the composition dependence of the InGaN bandgap and the size of the InN gap remain uncertain. We apply a combination of techniques, Electron Probe Microanalysis (X-ray fluorescence spectroscopy (XRF) in wavelength-dispersive mode) and Cathodoluminescence (CL) spectroscopy, to the first of these problems. Our method measures in situ the composition and the luminescence spectrum of almost coincident volumes of sample, of size about one cubic micron. The combination of microcomposition mapping with CL spectrum imaging produces very large E-P(x) datasets. (E-P is the peak energy of the emission band.) We discover an unexplained systematic difference in the E-P(x) dependences of samples grown by Molecular Beam Epitaxy and Metalorganic Vapour Phase Epitaxy with similar ranges of x from near zero to similar to0.4. The linear relationship previously established between the bandgap energy E-B, measured by absorption spectroscopy, and E-P for MOVPE samples allows an extrapolation of the MOVPE E-B(x) data to x = 1, representing pure InN, which yields a predicted gap of 0.7(1) eV. This is likely to underestimate the true value.},
keywords = {band gap, bowing parameter, InGaN, nitrides, semiconducting alloys, alloys, stokes shift, GAP, Inn, luminescence, localization, epilayers, emitters, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/30538/}
}

We have studied the optical properties of Eu doped GaN thin films. We have grown high quality Eu doped GaN thin films by using Gas Source Molecular Beam Epitaxy (GSMBE), with 1.4\% Eu concentration. The Full Width at Half Maximum (FWHM) of the X-ray diffraction in an omega scan was found to be 288 arcsecs. Low Eu concentration (0.08\%) doped GaN thin films were grown, where Eu-related photoluminescence at 622 and 613 nm was detected using above band-gap excitation at 2 K. For high Eu concentration of 30\% GaN:Eu crystal photoluminescence (PL) and cathodoluminescence (CL) spectra show strong and intense transitions at 622 and 664 nm, but also at 593 nm for CL spectra, with a similar transition observed from the low Eu concentration sample.

This presentation reviews recent lattice location studies of RE ions in GaN by electron emission channelling (EC) and X-ray absorption fine structure (XAFS) techniques. These studies agree that RE ions at low concentrations (whether they are incorporated during growth or introduced later by ion implantation) predominantly occupy Ga substitutional sites, as expected from considerations of charge equivalence. We combine this result with some examples of the welldocumented richness of optical spectra of GaN:RE3+ to suggest that the luminescence of these materials may be ascribed to a family of rather similar sites, all of which feature the REGa defect.

@article{strathprints3024,
volume = {831},
month = {December},
title = {Site multiplicity of rare earth ions in III-nitrides},
author = {K.P. O'Donnell and V. Katchkanov and K. Wang and R.W. Martin and B. Hourahine and P.R. Edwards and E. Nogales and J.F.W. Mosselmans and B. De-Vries},
year = {2004},
pages = {527--535},
journal = {MRS Online Proceedings Library},
keywords = {site multiplicity, rare earth ions, III-nitrides, nanoscience, Solid state physics. Nanoscience, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/3024/},
abstract = {This presentation reviews recent lattice location studies of RE ions in GaN by electron emission channelling (EC) and X-ray absorption fine structure (XAFS) techniques. These studies agree that RE ions at low concentrations (whether they are incorporated during growth or introduced later by ion implantation) predominantly occupy Ga substitutional sites, as expected from considerations of charge equivalence. We combine this result with some examples of the welldocumented richness of optical spectra of GaN:RE3+ to suggest that the luminescence of these materials may be ascribed to a family of rather similar sites, all of which feature the REGa defect.}
}

Light-emitting diodes (LEDs) based on an interconnected array of GaN/InGaN micro-ring elements have been demonstrated. The devices have electrical characteristics similar to those of conventional broad-area devices. However, due to the large surface areas provided by the sidewalls, the extraction efficiency is greatly enhanced. Intense light emission at the periphery of the micro-rings is observed upon excitation by an electron beam, suggesting scattering of the photons which are extracted through the sidewalls. The devices provide a doubling in total light output compared to a broad-area reference LED of equal light-generation area.

@article{strathprints3005,
volume = {83},
number = {22},
month = {March},
author = {H.W. Choi and M.D. Dawson and P.R. Edwards and R.W. Martin},
title = {High extraction efficiency InGaN micro-ring light emitting diodes},
journal = {Applied Physics Letters},
pages = {4483--4485},
year = {2004},
keywords = {light-emitting diodes, LEDs, GaN/InGaN micro-ring elements, nanoscience, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/3005/},
abstract = {Light-emitting diodes (LEDs) based on an interconnected array of GaN/InGaN micro-ring elements have been demonstrated. The devices have electrical characteristics similar to those of conventional broad-area devices. However, due to the large surface areas provided by the sidewalls, the extraction efficiency is greatly enhanced. Intense light emission at the periphery of the micro-rings is observed upon excitation by an electron beam, suggesting scattering of the photons which are extracted through the sidewalls. The devices provide a doubling in total light output compared to a broad-area reference LED of equal light-generation area.}
}

InxGa1?xN quantum dots have been fabricated by the selective growth of GaN micropyramid arrays topped with InGaN/GaN quantum wells. The spatially, spectrally, and time-resolved emission properties of these structures were measured using cathodoluminescence hyperspectral imaging and low-temperature microphotoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit decay lifetimes of approximately 0.5 ns, with linewidths down to 650 meV

Local density functional calculations are used to investigate models of the center responsible for a prominent set of luminescent lines with zero-phonon lines around 3.15 eV in hydrogen rich 4H-SiC and previously attributed to VSi-H. We find that the electronic structure of this defect and the character of its vibrational modes are inconsistent with this assignment. In contrast, a H2 * center, bound to a carbon anti-site, is more stable than the isolated molecule and possesses a donor level close to that observed for the H-lines. Moreover, its vibrational modes are in good agreement with experiment. A possible mechanism for the radiation enhanced quenching of the defect is discussed.

@Article{strathprints2964,
author = {D. Prezzi and T.A.G. Eberlein and R. Jones and B. Hourahine and P.R. Briddon and S. {\"O}berg},
title = {Hydrogen-related photoluminescent centers in SiC},
journal = {Physical Review B: Condensed Matter and Materials Physics},
year = {2004},
volume = {70},
number = {20},
pages = {205207},
month = {November},
abstract = {Local density functional calculations are used to investigate models of the center responsible for a prominent set of luminescent lines with zero-phonon lines around 3.15 eV in hydrogen rich 4H-SiC and previously attributed to VSi-H. We find that the electronic structure of this defect and the character of its vibrational modes are inconsistent with this assignment. In contrast, a H2 * center, bound to a carbon anti-site, is more stable than the isolated molecule and possesses a donor level close to that observed for the H-lines. Moreover, its vibrational modes are in good agreement with experiment. A possible mechanism for the radiation enhanced quenching of the defect is discussed.},
keywords = {local density functional calculations, photoluminescent centers, SiC, nanoscience, Solid state physics. Nanoscience, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/2964/}
}

An analysis of photoluminescence (PL) spectra for a range of single and multiple InGaN/GaN quantum wells, as a function of temperature, is presented. The well-known anomalous ‘S-shape’ behaviour is observed for samples emitting over a very wide range of energies. We present an analysis of this range of data, supplemented by other published data, in terms of different types of recombination sites within the wells. A quantitative model, based on previous work, to fit the temperature dependence of the emission peak energy is developed and gives good fits over an extended temperature range. The fitting parameters obtained are discussed in the light of values obtained from the literature and the intense piezoelectric fields present in the samples. Furthermore, the linewidth dependence of the PL emission peak energy in the region of the ‘S-shape’ is also analysed.

@Article{strathprints22217,
author = {R Pecharroman-Gallego and R W Martin and I M Watson},
title = {Investigation of the unusual temperature dependence of InGaN/GaN quantum well photoluminescence over a range of emission energies},
journal = {Journal of Physics D: Applied Physics},
year = {2004},
volume = {37},
number = {21},
pages = {2954--2961},
month = {November},
abstract = {An analysis of photoluminescence (PL) spectra for a range of single and multiple InGaN/GaN quantum wells, as a function of temperature, is presented. The well-known anomalous 'S-shape' behaviour is observed for samples emitting over a very wide range of energies. We present an analysis of this range of data, supplemented by other published data, in terms of different types of recombination sites within the wells. A quantitative model, based on previous work, to fit the temperature dependence of the emission peak energy is developed and gives good fits over an extended temperature range. The fitting parameters obtained are discussed in the light of values obtained from the literature and the intense piezoelectric fields present in the samples. Furthermore, the linewidth dependence of the PL emission peak energy in the region of the 'S-shape' is also analysed.},
keywords = {carrier dynamics, room-temperature, band, luminescence, epilayers, shift, laser, dimensionality, recombination, localization, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/22217/}
}

Gallium nitride based structures have been characterised using the novel approach of simultaneous wavelength-dispersive X-ray microanalysis and cathodoluminescence spectral mapping (or hyperspectral imaging). Details are presented of the instrumentation developed to carry out such measurements. Application of the technique to MOVPE-grown indium gallium nitride epilayers shows microscopic variations in the indium content, which correlate directly with spatially-dependent shifts observed in the peak wavelength of the luminescence spectrum. Regions of higher indium content are shown to emit at lower energy and with decreased intensity, mirroring equivalent macroscopic observations.

Micro-light emitting diode (LED) arrays with diameters of 4 to 20 mum have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output.

@article{strathprints5231,
volume = {93},
number = {10},
month = {May},
author = {H.W. Choi and C.W. Jeon and M.D. Dawson and P.R. Edwards and S. Tripathy and R.W. Martin},
title = {Mechanism of enhanced light output in InGaN-based microlight emitting diodes},
journal = {Journal of Applied Physics},
pages = {5978--5982},
year = {2003},
keywords = {indium compounds, gallium compounds, light emitting diodes, quantum well devices, semiconductor quantum wells, III-V semiconductors, wide band gap semiconductors, Raman spectra, cathodoluminescence, electroluminescence, optical microscopy, Optics. Light, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/5231/},
abstract = {Micro-light emitting diode (LED) arrays with diameters of 4 to 20 mum have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output.}
}

High-performance, two-dimensional arrays of parallel-addressed InGaN blue micro-light-emitting diodes (LEDs) with individual element diameters of 8, 12, and 20 /spl mu/m, respectively, and overall dimensions 490 /spl times/490 /spl mu/m, have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallization, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have current-voltage (I-V) characteristics approaching those of broad-area reference LEDs fabricated from the same wafer, and give comparable (3-mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction.

Gallium nitride based structures have been characterised using the novel approach of simultaneous wavelength-dispersive X-ray microanalysis and cathodoluminescence spectral mapping. Details are presented of the instrumentation developed to carry out such measurements. Application of the technique to MOVPE-grown indium gallium nitride epilayers shows microscopic variations in the indium content, which correlate directly with spatially-dependent shifts observed in the peak wavelength of the luminescence spectrum. Regions of higher indium content are shown to emit at lower energy, mirroring equivalent macroscopic observations

All III-N visible light emitting devices contain ultrathin active layers of InGaN. Although this material has been widely studied during the last ten years or so, opinion is still divided as to its nature. Most researchers would agree with the proposition that III-nitride "alloys" are a mess, at least when compared with analogous III-As materials. It may be further argued that the quality of InGaN samples is at present too variable to allow general statements to be made about the material. We repudiate this misconception. The similarities between luminescent InGaN samples from different laboratories outweigh the differences. Any differences that do occur can be confidently accounted for, in terms of a peculiar growth habit of III-nitrides. We also briefly discuss the status of accidental InN quantum dots.

@article{strathprints30541,
volume = {195},
number = {3},
month = {February},
author = {K P O'Donnell and S Pereira and R W Martin and P R Edwards and M J Tobin and J F W Mosselmans},
title = {Wishful physics: Some common misconceptions about InGaN},
journal = {Physica Status Solidi A - Applications and Materials Science},
pages = {532--536},
year = {2003},
keywords = {photoluminescence excitation spectroscopy, absorption fine-structure, molecular-beam epitaxy, X-ray-diffraction, exciton localization, quantum-wells, stokes shift, epilayers, layers, dependence, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/30541/},
abstract = {All III-N visible light emitting devices contain ultrathin active layers of InGaN. Although this material has been widely studied during the last ten years or so, opinion is still divided as to its nature. Most researchers would agree with the proposition that III-nitride "alloys" are a mess, at least when compared with analogous III-As materials. It may be further argued that the quality of InGaN samples is at present too variable to allow general statements to be made about the material. We repudiate this misconception. The similarities between luminescent InGaN samples from different laboratories outweigh the differences. Any differences that do occur can be confidently accounted for, in terms of a peculiar growth habit of III-nitrides. We also briefly discuss the status of accidental InN quantum dots.}
}

Rare earth doped GaN structures offer potential for optical devices emitting in the visible spectral region. Doping with europium and erbium produces characteristic sharp red and green emission lines respectively, due to intra-4f(n) shell electron transitions. We describe studies of Eu- and Er- ion implanted MOCVD grown GaN epilayers on sapphire using an electron probe micro-analyser modified to allow cathodoluminescence (CL) spectroscopy. Wavelength dispersive X-ray analysis is shown to be an accurate technique for quantifying rare earth concentrations in GaN, down to similar to0.06 atomic \% and is complemented by CL acquired at the same time from the same microscopic region of sample.

The rovibrational-translational states of a hydrogen molecule moving in a cage site in Si, when subjected to an electrical field arising from its surroundings, are investigated. The wave functions are expressed in terms of basis functions consisting of the eigenfunctions of the molecule confined to move in the cavity and rovibrational states of the free molecule. The energy levels, intensities of infrared and Raman transitions, effects of uniaxial stress, and a neighboring oxygen defect are found and compared with existing experimental data.

@Article{strathprints2965,
author = {B. Hourahine and R. Jones},
title = {Infrared activity of hydrogen molecules trapped in Si},
journal = {Physical Review B: Condensed Matter and Materials Physics},
year = {2003},
volume = {67},
number = {12},
pages = {121205(R)},
month = {March},
abstract = {The rovibrational-translational states of a hydrogen molecule moving in a cage site in Si, when subjected to an electrical field arising from its surroundings, are investigated. The wave functions are expressed in terms of basis functions consisting of the eigenfunctions of the molecule confined to move in the cavity and rovibrational states of the free molecule. The energy levels, intensities of infrared and Raman transitions, effects of uniaxial stress, and a neighboring oxygen defect are found and compared with existing experimental data.},
keywords = {infrared, hydrogen molecules, Si, electrical field, Raman transitions, nanoscience, Solid state physics. Nanoscience, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/2965/}
}

Highly efficient, two-dimensional arrays of parallel-addressed InGaN blue micro-LEDs with individual element diameters of 8, 12 and 20microns have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallisation, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have I-V characteristics similar to those of broad-area reference LEDs fabricated from the same wafer, and give superior (3mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction.

@InCollection{strathprints9008,
author = {H.W. Choi and C.W. Jeon and M.D. Dawson and P.R. Edwards and R.W. Martin},
title = {Efficient GaN-based micro-LED arrays},
booktitle = {Proceedings of the 2002 MRS Fall Meeting},
publisher = {MRS},
year = {2002},
editor = {Christian Wetzel and Edward T. Yu and James S. Speck and Angela Rizzi and Yasuhiko Arakawa},
volume = {743},
pages = {433--438},
note = {AHR},
abstract = {Highly efficient, two-dimensional arrays of parallel-addressed InGaN blue micro-LEDs with individual element diameters of 8, 12 and 20microns have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallisation, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have I-V characteristics similar to those of broad-area reference LEDs fabricated from the same wafer, and give superior (3mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction.},
keywords = {InGaN, blue micro-LED, fabrication, efficiencies, Optics. Light},
url = {http://strathprints.strath.ac.uk/9008/}
}

The fabrication and performance of GaN-based micro-light emitting diode (g-LED) arrays with 64{$\times$}64 elements is reported. The diameter of each element is 20 {\ensuremath{\mu}}m and center-to-center spacing of 30 {\ensuremath{\mu}}m, giving an overall active area of the arrays of 80425 {\ensuremath{\mu}}m2. With the introduction of a matrix addressing scheme, the number of bond pads has been reduced from n2 to 2n, facilitating the packaging of the devices. The arrays emit {\ensuremath{>}}50 {\ensuremath{\mu}}W per element at 3mA drive current. The advances in fabrication and performance reported in this work bring these arrays to the threshold of advanced research and commercial applications in areas of micro-displays, sensing, biophotonics and data-communications.

@incollection{strathprints9007,
volume = {2},
month = {November},
author = {C.W. Jeon and H.W. Choi and P.R. Edwards and A.C. Bryce and M.D. Dawson},
booktitle = {Lasers and Electro-Optics Society, 2002. LEOS 2002. The 15th Annual Meeting of the IEEE},
title = {64 x 64 matrix-addressable arrays of GaN-based micro-LEDs},
publisher = {IEEE},
pages = {685--686},
year = {2002},
keywords = {LED displays, gallium compounds, light emitting diodes, micro-optics, microdisplays, optical arrays, semiconductor device packaging, wide band gap semiconductors, Optics. Light},
url = {http://strathprints.strath.ac.uk/9007/},
abstract = {The fabrication and performance of GaN-based micro-light emitting diode (g-LED) arrays with 64{$\times$}64 elements is reported. The diameter of each element is 20 {\ensuremath{\mu}}m and center-to-center spacing of 30 {\ensuremath{\mu}}m, giving an overall active area of the arrays of 80425 {\ensuremath{\mu}}m2. With the introduction of a matrix addressing scheme, the number of bond pads has been reduced from n2 to 2n, facilitating the packaging of the devices. The arrays emit {\ensuremath{>}}50 {\ensuremath{\mu}}W per element at 3mA drive current. The advances in fabrication and performance reported in this work bring these arrays to the threshold of advanced research and commercial applications in areas of micro-displays, sensing, biophotonics and data-communications.}
}

We report a detailed compositional analysis of InxGa1-xN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. Depth profiles of the InN fraction, x, in the MQWs were determined from grazing incidence Rutherford backscattering spectroscopy (RBS) analysis. Simulation of the RBS spectra provides precise estimations of individual well compositions, thickness, and the extent of In/Ga intermixing. It is ascertained that intermixing, and In segregation to the GaN cap layer, strongly increase with the value of x in the wells and with the number of periods in the MQW stack. Deleterious effects of intermixing on the spectral properties are apparent when comparing the photoluminescence spectra of two MQW structures with 8 and 18 wells, grown under the same nominal conditions.

@Article{strathprints38693,
author = {de Sousa Pereira, S. M. and E.M. Ferreira Pereira Lopes and E. Alves and N.P. Barradas and K.P. O'Donnell and C. Liu and C.J. Deatcher and I.M. Watson},
title = {Depth profiling InGaN/GaN multiple quantum wells by Rutherford backscattering: the role of intermixing},
journal = {Applied Physics Letters},
year = {2002},
volume = {81},
number = {16},
pages = {2950--2952},
abstract = {We report a detailed compositional analysis of InxGa1-xN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. Depth profiles of the InN fraction, x, in the MQWs were determined from grazing incidence Rutherford backscattering spectroscopy (RBS) analysis. Simulation of the RBS spectra provides precise estimations of individual well compositions, thickness, and the extent of In/Ga intermixing. It is ascertained that intermixing, and In segregation to the GaN cap layer, strongly increase with the value of x in the wells and with the number of periods in the MQW stack. Deleterious effects of intermixing on the spectral properties are apparent when comparing the photoluminescence spectra of two MQW structures with 8 and 18 wells, grown under the same nominal conditions.},
keywords = {quantum wells , Rutherford backscattering, intermixing, Optics. Light, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/38693/}
}

A-plane InxGa1?xN/GaN (x = 0.09, 0.14, 0.24, and 0.3) multiple-quantum-wells (MQWs) samples, with a well width of about 4.5 nm, were achieved by utilizing r-plane sapphire substrates. Optical quality was investigated by means of photoluminescence (PL), cathodoluminescence, and time resolved PL measurements (TRPL). Two distinguishable emission peaks were examined from the low temperature PL spectra, where the high- and low-energy peaks were ascribed to quantum wells and localized states, respectively. Due to an increase in the localized energy states and absence of quantum confined Stark effect, the quantum efficiency was increased with increasing indium composition up to 24\%. As the indium composition reached 30\%, however, pronounced deterioration in luminescence efficiency was observed. The phenomenon could be attributed to the high defect densities in the MQWs resulted from the increased accumulation of strain between the InGaN well and GaN barrier. This argument was verified from the much shorter carrier lifetime at 15 K and smaller activation energy for In0.3Ga0.7N/GaN MQWs. In addition, the polarization-dependent PL revealed that the degree of polarization decreased with increasing indium compositions because of the enhancement of zero-dimensional nature of the localizing centers. Our detailed investigations indicate that the indium content in a-plane InGaN/GaN MQWs not only has an influence on optical performance, but is also important for further application of nitride semiconductors.

@Article{strathprints37280,
author = {de Sousa Pereira, S. M. and M.R. Correia and E. Pereira and K.P. O'Donnell and E. Alves and N.P. Barradas and A.D. Sequeira and I.M. Watson and C. Liu},
title = {Degradation of structural and optical properties of InGaN/GaN multiple quantum wells},
journal = {Journal of Applied Physics},
year = {2002},
volume = {105},
pages = {302--306},
abstract = {A-plane InxGa1?xN/GaN (x = 0.09, 0.14, 0.24, and 0.3) multiple-quantum-wells (MQWs) samples, with a well width of about 4.5 nm, were achieved by utilizing r-plane sapphire substrates. Optical quality was investigated by means of photoluminescence (PL), cathodoluminescence, and time resolved PL measurements (TRPL). Two distinguishable emission peaks were examined from the low temperature PL spectra, where the high- and low-energy peaks were ascribed to quantum wells and localized states, respectively. Due to an increase in the localized energy states and absence of quantum confined Stark effect, the quantum efficiency was increased with increasing indium composition up to 24\%. As the indium composition reached 30\%, however, pronounced deterioration in luminescence efficiency was observed. The phenomenon could be attributed to the high defect densities in the MQWs resulted from the increased accumulation of strain between the InGaN well and GaN barrier. This argument was verified from the much shorter carrier lifetime at 15 K and smaller activation energy for In0.3Ga0.7N/GaN MQWs. In addition, the polarization-dependent PL revealed that the degree of polarization decreased with increasing indium compositions because of the enhancement of zero-dimensional nature of the localizing centers. Our detailed investigations indicate that the indium content in a-plane InGaN/GaN MQWs not only has an influence on optical performance, but is also important for further application of nitride semiconductors.},
keywords = {optical properties , quantum physics, quantum wells, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/37280/}
}

In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite InxGa1?xN/GaN epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for InxGa1?xN/GaN structures was calculated as a function of the InN content, x, using the energy balance model proposed by People and Bean [Appl. Phys. Lett. 47, 322 (1985)]. Experimentally determined CLT are in good agreement with these calculations. The occurrence of discontinuous strain relaxation (DSR), when the CLT is exceeded, is revealed in the case of a 120 nm thick In0.19Ga0.89N layer by x-ray reciprocal space mapping of an asymmetrical reflection. The effect of DSR on the luminescence of this layer is clear: The luminescence spectrum shows two peaks centered at {$\sim$}2.50 and {$\sim$}2.67 eV, respectively. These two components of the luminescence of the sample originate in regions of different strain, as discriminated by depth-resolving cathodoluminescence spectroscopy. DSR leads directly to the emergence of the second, lower-energy, peak. Based on this experimental evidence, it is argued that the appearance of luminescence doublets in InGaN is not evidence of ?quantum dotlike In-rich? or ?phase separated? regions, as commonly proposed.

@Article{strathprints31891,
author = {de Sousa Pereira, Sergio Manuel and M.R. Correia and Eduarda Pereira and Carol Trager-Cowan and Francis Sweeney and Kevin O'Donnell and E. Alves and N. Franco and A.D. Sequeira},
title = {Structural and optical properties of InGaN/GaN layers close to the critical layer thickness},
journal = {Applied Physics Letters},
year = {2002},
volume = {81},
number = {7},
pages = {1207},
month = {June},
abstract = {In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite InxGa1?xN/GaN epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for InxGa1?xN/GaN structures was calculated as a function of the InN content, x, using the energy balance model proposed by People and Bean [Appl. Phys. Lett. 47, 322 (1985)]. Experimentally determined CLT are in good agreement with these calculations. The occurrence of discontinuous strain relaxation (DSR), when the CLT is exceeded, is revealed in the case of a 120 nm thick In0.19Ga0.89N layer by x-ray reciprocal space mapping of an asymmetrical reflection. The effect of DSR on the luminescence of this layer is clear: The luminescence spectrum shows two peaks centered at {$\sim$}2.50 and {$\sim$}2.67 eV, respectively. These two components of the luminescence of the sample originate in regions of different strain, as discriminated by depth-resolving cathodoluminescence spectroscopy. DSR leads directly to the emergence of the second, lower-energy, peak. Based on this experimental evidence, it is argued that the appearance of luminescence doublets in InGaN is not evidence of ?quantum dotlike In-rich? or ?phase separated? regions, as commonly proposed.},
keywords = {indium compounds, gallium compounds, cathodoluminescence , semiconductor epitaxial layers, Physics, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/31891/}
}

In this paper we describe the use of electron backscattered diffraction (EBSD) for the characterisation of nitride thin films, and report its use in the study of the spatial variation of strain across an epitaxially laterally overgrown GaN (ELOG) thin film. We also discuss the combination of luminescence and EBSD measurements to enable luminescence properties of samples to be directly correlated with their crystallographic properties. We compare photoluminescence spectra and EBSD measurements from a set of GaN thin films grown on off-axis sapphire substrates, revealing the tilt of a GaN thin film grown on a 10? off-axis sapphire substrate to be responsible for the observation of luminescence defect bands in this film. We finally report on the use of EBSD to identify zinc blende regions in a predominantly wurtzite MBE film, with cathodoluminescence used to obtain correlated luminescence spectra.

@article{strathprints3087,
volume = {0},
number = {1},
month = {December},
author = {C. Trager-Cowan and F. Sweeney and A.J. Wilkinson and I.M. Watson and P.G. Middleton and K.P. O'Donnell and D. Zubia and S.D. Hersee and S. Einfeldt and D. Hommel},
title = {Determination of the structural and luminescence properties of nitrides using electron backscattered diffraction and photo- and cathodoluminescence},
journal = {Physica Status Solidi C},
pages = {532--536},
year = {2002},
keywords = {luminescence, nitrides, electron backscattered diffraction, cathodoluminescence, nanoscience, Solid state physics. Nanoscience, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/3087/},
abstract = {In this paper we describe the use of electron backscattered diffraction (EBSD) for the characterisation of nitride thin films, and report its use in the study of the spatial variation of strain across an epitaxially laterally overgrown GaN (ELOG) thin film. We also discuss the combination of luminescence and EBSD measurements to enable luminescence properties of samples to be directly correlated with their crystallographic properties. We compare photoluminescence spectra and EBSD measurements from a set of GaN thin films grown on off-axis sapphire substrates, revealing the tilt of a GaN thin film grown on a 10? off-axis sapphire substrate to be responsible for the observation of luminescence defect bands in this film. We finally report on the use of EBSD to identify zinc blende regions in a predominantly wurtzite MBE film, with cathodoluminescence used to obtain correlated luminescence spectra.}
}

Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light-emitting diodes and laser diodes. However, because of the lattice mismatch between presently used substrates and epitaxial nitride thin films, the films are of variable quality. In this paper we describe our preliminary studies of nitride thin films using electron backscattered diffraction (EBSD). We show that the EBSD technique may be used to reveal the relative orientation of an epitaxial thin film with respect to its substrate (a 90? rotation between a GaN epitaxial thin film and its sapphire substrate is observed) and to determine its tilt (a GaN thin film was found to be tilted by 13{$\pm$}1? towards [10 0]GaN), where the tilt is due to the inclination of the sapphire substrate(cut off-axis by 10? from (0001)sapphire towards (10 0)sapphire). We compare EBSD patterns obtained from As-doped GaN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) with low and high As 4 flux, respectively. Higher As 4 flux results in sharper, better defined patterns, this observation is consistent with the improved surface morphology observed in AFM studies. Finally, we show that more detail can be discerned in EBSD patterns from GaN thin films when samples are cooled.

@article{strathprints3086,
volume = {205},
number = {3},
month = {March},
author = {C. Trager-Cowan and F. Sweeney and J. Hastie and S.K. Manson-Smith and D.A. Cowan and D. McColl and A. Mohammed and K.P. O'Donnell and D. Zubia and S.D. Hersee and C.T. Foxon and I. Harrison and S.V. Novikov},
title = {Characterisation of nitride thin films by EBSD},
journal = {Journal of Microscopy},
pages = {226--230},
year = {2002},
keywords = {crystalline quality, diffraction, EBSD, electron backscatterdiffraction, epitaxial thin films, GaN, Kikuchi, nitride thin films, sapphire, temperature dependence, nanoscience, Solid state physics. Nanoscience, Pathology and Forensic Medicine, Histology},
url = {http://strathprints.strath.ac.uk/3086/},
abstract = {Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light-emitting diodes and laser diodes. However, because of the lattice mismatch between presently used substrates and epitaxial nitride thin films, the films are of variable quality. In this paper we describe our preliminary studies of nitride thin films using electron backscattered diffraction (EBSD). We show that the EBSD technique may be used to reveal the relative orientation of an epitaxial thin film with respect to its substrate (a 90? rotation between a GaN epitaxial thin film and its sapphire substrate is observed) and to determine its tilt (a GaN thin film was found to be tilted by 13{$\pm$}1? towards [10 0]GaN), where the tilt is due to the inclination of the sapphire substrate(cut off-axis by 10? from (0001)sapphire towards (10 0)sapphire). We compare EBSD patterns obtained from As-doped GaN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) with low and high As 4 flux, respectively. Higher As 4 flux results in sharper, better defined patterns, this observation is consistent with the improved surface morphology observed in AFM studies. Finally, we show that more detail can be discerned in EBSD patterns from GaN thin films when samples are cooled.}
}

Strain and composition distributions within wurtzite InGaN/GaN layers are investigated by high-resolution reciprocal space mapping (RSM). We illustrate the potential of RSM to detect composition and strain gradients independently. This information is extracted from the elongation of broadened reciprocal lattice points (RLP) in asymmetric x-ray reflections. Three InxGa12xN/GaN (nominal x50.25) samples with layer thickness of 60, 120, and 240 nm, were grown in a commercial metal-organic chemical vapor deposition reactor. The RSMs around the (105) reflection show that the strain profile is nonuniform over depth in InGaN. The directions of ”pure” strain relaxation in the reciprocal space, for a given In content (isocomposition lines), are calculated based on elastic theory. Comparison between these directions and measured distributions of the RLP shows that the relaxation process does not follow a specific isocomposition line. The In mole fraction (x) increases as the films relax. At the start of growth all the films have x;0.2 and are coherent to GaN. As they relax, x progressively increases towards the nominal value (0.25). Compositional gradients along the growth direction extracted from the RSM analysis are confirmed by complementary Rutherford backscattering measurements.

@article{strathprints3023,
volume = {80},
number = {21},
month = {May},
author = {S. Pereira and M.R. Correia and E. Pereira and K.P. O'Donnell and E. Alves and A.D. Sequeira and N. Franco and I.M. Watson and C.J. Deatcher},
title = {Strain and composition distributions in wurtzite InGaN/GaN layers extracted from x-ray reciprocal space mapping},
journal = {Applied Physics Letters},
pages = {3913--3915},
year = {2002},
keywords = {wurtzite InGaN/GaN layers, x-ray reciprocal space mapping, nanoscience, space mapping, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/3023/},
abstract = {Strain and composition distributions within wurtzite InGaN/GaN layers are investigated by high-resolution reciprocal space mapping (RSM). We illustrate the potential of RSM to detect composition and strain gradients independently. This information is extracted from the elongation of broadened reciprocal lattice points (RLP) in asymmetric x-ray reflections. Three InxGa12xN/GaN (nominal x50.25) samples with layer thickness of 60, 120, and 240 nm, were grown in a commercial metal-organic chemical vapor deposition reactor. The RSMs around the (105) reflection show that the strain profile is nonuniform over depth in InGaN. The directions of ''pure'' strain relaxation in the reciprocal space, for a given In content (isocomposition lines), are calculated based on elastic theory. Comparison between these directions and measured distributions of the RLP shows that the relaxation process does not follow a specific isocomposition line. The In mole fraction (x) increases as the films relax. At the start of growth all the films have x;0.2 and are coherent to GaN. As they relax, x progressively increases towards the nominal value (0.25). Compositional gradients along the growth direction extracted from the RSM analysis are confirmed by complementary Rutherford backscattering measurements.}
}

The electrical activity of Cs-H defects in Si has been investigated in a combined modeling and experimental study. High-resolution Laplace capacitance spectroscopy with the uniaxial stress technique has been used to measure the stress-energy tensor and the results are compared with theoretical modeling. At low temperatures, implanted H is trapped as a negative-U center with a donor level in the upper half of the gap. However, at higher temperatures, H migrates closer to the carbon impurity and the donor level falls, crossing the gap. At the same time, an acceptor level is introduced into the upper gap making the defect a positive-U center.

@Article{strathprints2966,
author = {O. Andersen and A.R. Peaker and L. Dobaczewski and K. Bonde Nielsen and B. Hourahine and R. Jones and P.R. Briddon and S. {\"O}berg},
title = {Electrical activity of carbon-hydrogen centers in Si},
journal = {Physical Review B: Condensed Matter and Materials Physics},
year = {2002},
volume = {66},
number = {23},
pages = {235205},
month = {December},
abstract = {The electrical activity of Cs-H defects in Si has been investigated in a combined modeling and experimental study. High-resolution Laplace capacitance spectroscopy with the uniaxial stress technique has been used to measure the stress-energy tensor and the results are compared with theoretical modeling. At low temperatures, implanted H is trapped as a negative-U center with a donor level in the upper half of the gap. However, at higher temperatures, H migrates closer to the carbon impurity and the donor level falls, crossing the gap. At the same time, an acceptor level is introduced into the upper gap making the defect a positive-U center.},
keywords = {electrical activity, carbon-hydrogen centers, Si, nanoscience, Solid state physics. Nanoscience, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/2966/}
}

In this paper, we describe the growth and characterization of InGaN single quantum wells with emission peaks in the blue, green, amber and red spectral regions, grown by metal-organic vapour phase epitaxy. Starting from the growth of a blue-emitting (peak {\texttt{\char126}}430 nm) InGaN quantum well at 860?C the InGaN growth temperature was progressively reduced. The photoluminescence peak wavelength, measured at low temperature, shifts through the green and orange spectral regions and reaches 670 nm for an InGaN growth temperature of 760?C. This corresponds to an energy lower than the currently accepted band-gap of the binary compound, InN. Spectral characteristics of the luminescence peaks will be discussed, including an analysis of the phonon-assisted contribution. Low energy secondary ion mass spectrometry analysis provides information on the indium content and thickness of the ‘blue’ and ‘red’ quantum wells. The results are combined to discuss the origin of the ‘sub-band-gap’ luminescence in terms of the combined influence of InN-GaN segregation and the effect of intense piezoelectric fields

@article{strathprints23447,
volume = {35},
number = {7},
month = {April},
author = {R.W. Martin and P.R. Edwards and R. Pecharroman-Gallego and C. Liu and C.J. Deatcher and I.M. Watson and K.P. O'Donnell},
title = {Light emission ranging from blue to red from a series of InGaN/GaN single quantum wells},
journal = {Journal of Physics D: Applied Physics},
pages = {604--608},
year = {2002},
keywords = {condensed matter, electrical, magnetic and optical, Semiconductors, surfaces, interfaces, thin films, nanoscale science and low-D systems, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/23447/},
abstract = {In this paper, we describe the growth and characterization of InGaN single quantum wells with emission peaks in the blue, green, amber and red spectral regions, grown by metal-organic vapour phase epitaxy. Starting from the growth of a blue-emitting (peak {\texttt{\char126}}430 nm) InGaN quantum well at 860?C the InGaN growth temperature was progressively reduced. The photoluminescence peak wavelength, measured at low temperature, shifts through the green and orange spectral regions and reaches 670 nm for an InGaN growth temperature of 760?C. This corresponds to an energy lower than the currently accepted band-gap of the binary compound, InN. Spectral characteristics of the luminescence peaks will be discussed, including an analysis of the phonon-assisted contribution. Low energy secondary ion mass spectrometry analysis provides information on the indium content and thickness of the 'blue' and 'red' quantum wells. The results are combined to discuss the origin of the 'sub-band-gap' luminescence in terms of the combined influence of InN-GaN segregation and the effect of intense piezoelectric fields}
}

Photoluminescence (PL) has been reported from InGaN-based heterostructures, including thick epilayers on GaN, InGaN/GaN quantum wells and InGaN/GaN quantum boxes, with peak energies ranging from 3.44 to 1.31 eV at low temperature. The corresponding absorption spectra are not always easy to obtain, but photoluminescence excitation (PLE) spectroscopy provides an efficient means of obtaining comparable information. We describe here a comprehensive investigation of PLE spectra from a wide range of InGaN samples. Variation of the measured bandgap energy with the detection energy for individual samples suggests that the InGaN emission spectrum is inhomogeneously broadened. The PLE spectrum obtained at the peak emission energy of a particular sample is equivalent to the absorption spectrum of that sample. The data range of the band edge measurements is extended to lower energies by the PLE results. In general, the PLE data confirm the existence of a linear relationship between the optical bandgap and the emission energy.